Had no plans to review this despite my love for it but it's currently free to own permanently, for a full week at the time of writing so I'm obligated to shill. Sorry in advance.

It's an unspoken rule in my social circles that you shouldn't play 4x games with me, especially Civilization.

Why?

I like war. I consider it the sole reason to play these games. Managing an economy and production in such a way that it allows you to continue the maintenance of an ever-expanding warfront is always where I get most of my fun from, and honestly I think it's where the mechanics of 4x games really come together as a whole. Warfare is expensive, after all, and building to keep it afloat is honestly more engaging than building farms or whatever- God that makes me sound like a US politician.

But I grew tired of warfare in 4x games, Civ-likes especially. Past a certain point they're little more than "right click doomstack onto enemy until dead", forever. I longed for a game in this style that had more strategic depth to it, and ideally none of that cowardly shit like "turtling" or "pacifism". I wanted units that countered one another in a vicious cycle, and factions that felt meaningfully different beyond having passives like "The French enforced unfair amounts of debt on Haiti when they revolted agains French rule" or "The English get +2 to xenophobia" alongside maybe one different unit.

Gladius was the answer to that prayer.

On the surface this game isn't very appealing. To put it bluntly: Gladius is an ugly game. The UI is straightforward and no-frills, the music doesn't ever peak above "serviceable", the models are straight forward and lo-fi in not particularly eye-catching ways, and the terrain looks fairly uninteresting regardless of place. This is the brownest game to come out of the 2010s, somehow.

The onboarding process isn't much better, not doing much besides giving you a basic overview of the controls and telling you how to attack. If you let your Civ instincts kick in, you're almost certainly going to get killed by the trash mobs rather than any enemy players. I have personally sat in voice chat and watched as people get washed by trying to play this like Civ 6.

Gladius is not Civ, nor is it Endless Legend or Humankind or whatever Civ-likes the youth enjoy these days.

The big difference, I feel, is in the units.

Every Gladius faction has their own unique units, but there's ultimately not that many of them and you don't always unlock straight up "better" units. Each unit comes with inherent traits, but also weapons which have their own traits as well as accuracy/armor piercing/etc stats. Starter units tend to be all-rounders, but later ones are more specialized.
In contrast to Civ, Endless Legend or even Age of Wonder, units don't often become redundant in Gladius. Indeed, 'science' in this game is little more than unlocking more units and more abilities for units.
It's these extra abilities that add a lot to the tactical nature of combat. Take the standard Space Marine unit - the Tactical Marine - for example. They're not very flashy, having nothing more than their Bolter for combat, but research gives them several grenade types that let them hold their own against armored units, vehicles, or monstrous units.
Similarly, the design of the game is such that there's an ever-growing chain of counters to consider. It may seem alluring to ignore 'trash' units and field tons of tanks and uber units, but they can so easily be felled by focus fire, anti-armor auxiliary abilities, and flanking.

Ah, the morale system. Combat's most important addition.

Gladius uses the standard grid-based system of Civ and its imitators, and naturally this means units occupy one tile each while being unable to share tiles. The map generation is scarily good at creating chokepoints, dense foliage/ruins that obscure sight, and advantageous terrain. If units are flanked, lose too many allies in a single turn, or take attacks they can't retaliate against, they'll eventually start to crack. At first they'll simply take a minor stat penalty, but this eventually snowballs until they're taking obscene damage and dealing paltry damage.
More than any game of its kind, I feel, Gladius deeply incentivizes the need to have a solid frontline, but also some support. It may seem sensible to bundle your units up, but it only takes one teleporting/jetpacking/longjumping unit to break that formation and start draining your morale.
Terrain and engagement become more meaningful. Cliffs confer an incredible advantage against melee units, city ruins and trees interrupt the accuracy and effectiveness of long range units, and while Space Marines can attack from two tiles away, they get a bonus for being up close which necessitates a moderate amount of risk-taking - to name one example of how traits dictate fights.

Hero units are also present and while they're not quite ultra demi-gods who can solo entire armies, they provide enough buffs and offensive edge that slotting them into your formations becomes a game unto itself. They can level up alongside your trash unit, but you get to pick perks for them. These perks are, honestly, I'd say the only part of the game on a design level that kinda sucks; there's always an ideal route to take them and a lot of are just outright doodoo. Fortunately, heroes aren't mandatory, so I don't hate this too much.

I'm not gonna go through every faction's gameplan, unique traits, and design philosophy. There's way too many of them, and nothing is shared. Even the Chaos Space Marines diverge from their brethren significantly.

By now I assume you've looked at the store page and seen the enormous amount of DLC. Let's chat about that.

Is any of it mandatory? No. Proxy Studios keep their game meticulously balanced, with patches every few weeks. Gladius is perfectly playable without any DLC, and for the longest time I didn't own anything besides the Chaos Space Marine DLC.
But it is substantial. Those packs add a lot of options to each race, and paradoxically their value diminishes the less races you have. The latest unit pack, released alongside the free-to-claim weekend, has 11 units but 7 are inaccessible if you own no DLC. These aren't P2W unit packs, but the units do offer new tactical avenues that make for more interesting gameplay. To tie both topics together: In the latest pack, and after 6 years of existence, Assault Marines (teleporting melee) finally got an upgrade in the form of Assault Terminators, which is huge for the survivability of what're usually disposable shock units.
Personally, I'd just buy whatever tickles your fancy. I'm a crazy person and one of this game's strongest shooters, so I have all of it.

Gladius isn't an easy game to love, and I wager 90% of people who try it will bounce off because it's very aggressively not like other 4x games. If you can stick with it, though, it provides a uniquely rewarding experience. Plus, for some arcane reason, it refuses to die. It gets two DLCs a year and it's lasted six entire years with Proxy promising to keep going.