Thematically, Space Marine does not take anything from gears, since... well, it came first. In terms of gameplay - there's not that much similarity between the two. Space Marine has no cover system, and although it has chainsaws, they work differently than the lancer in Gears. Aside from the occasional "During-talking-the-characters-will-walk-slow" moments, I can't really think of anything that similar between the two. They play very differently - go play Gears of War 2, then play Space Marine. You'll notice the difference straight away. One involves taking sitting behind a wall a lot, and leap/rolling around the battlefield away from enemies (unless you want to chainsaw them), and the other is just shoot-shoot-stab-shoot-shoot-stab. Both are fun, but they are not the same. If you tried to play gears like space marine, you'd die even on the easiest setting. If you tried to play space marine like gears, you wouldn't get very far because you HAVE to melee kill in order to get your health back (you can use you fury thingamajig as well, but most of them you'll have to stun and kill a dude to get the health back). The universe of Warhammer 40000 and Gears are also very different - both are military themed, but one is about a bunch of genetically modified freaks running around whilst being fascist jerks to everyone and everything (the Imperium of Man are NOT the good guys, something I feel Space Marine did not do enough to get across - the Space Marines aren't actually heroes, as they are portrayed in the game, but are in fact jack-booted super soldiers who run around doing whatever they're told by their masters), and the other is about action heroes fighting for survival against seemingly impossible odds.
There's really NOT that much in common between the two aside from burly dudes and chainsaws.