Q:If I give my opponent's Infinite / Infinite Bramblesnap Spiritualize (whenever it deals damage, I gain that much life) am I dead when it hits, or do I come out at infinite life?(disregarding the fact that you can't do actual infinities, it's just to convey the point)
Unfortunately, due to state-based actions, you lose before the Spiritualize ever hits the stack to do anything meaningful.
704.1. State-based actions are game actions that happen automatically whenever certain conditions (listed below) are met. State-based actions don’t use the stack.
704.5a If a player has 0 or less life, he or she loses the game.