Okay. Look, I get this.
If you look at the two games, you're going to see a lot of burly armored men shooting at toothy grunts, and the aesthetic of grit and impact is much the same between them. But here's where the point that the Warhammer IP predates Gears is relevant. When I played the original Gears of War in 2006, I had no idea what the guns or the enemies were called in-universe, because I just called them "bolters" and "orks". I refered to the game in conversation as "Gears of Warhammer" while nodding smugly. The games do have the same aesthetic; but it's Warhammer's aesthetic. 40K not only had it first, but is in fact characterized and known primarily as its owner. "Grimdark" = "Warhammer".
Alright? So after we get that out of the way, we talk about the gameplay. And I agree that even if the Warhammer IP came first, that's no reason to say that Space Marine the game couldn't have stolen from Gears.
But, look: Gears of War's gameplay is defined by its cover system; if it didn't invent it, it certainly codified it and did it so well that every game afterwards tried to copy it. This and the gritty aesthetic is everything that Gears of War is. Every other component is a convention of third-person shooters. The D-pad weapon select is identical to the one found in Just Cause 2, and is descended from similar systems in much older games, like the Gamecube-era James Bond shooters. Space Marine's sprint via clicking the left stick is actually a different control from Gears, and is something I first saw in Modern Warfare. These things are generic.
Gears of War is all about shooting from, hiding behind, and moving between cover. In contrast, most of the enemies in the singleplayer game charge right at you (and are kind of better at dodging and weaving than I'm used to seeing, even after tons of Left4Dead), forcing you to engage with your melee weapon in a way totally divergent from anything in Gears. The balance of your time with this game is meant to be played surrounded by enemies, not at range. The game does not even let you crouch. You are not in a slow and grim advance, fighting desperately for survival. You are engaging in GLORIOUS BATTLE.