You'd have to be pretty new to multiplayer games or gaming in general to think that spawn-trapping and rage quitting don't exist. Neither concept is new nor are they exclusive to GTA Online.
Being spawn-trapped generally means being repetitively killed quickly or otherwise being unable to get very far after spawning. It doesn't matter if I can get to cover, or even if I can shoot back; if I can't get very far out of my spawn area without getting killed, or at all, I'm being spawn-trapped. Could I shoot my way out? Of course.. depending on mine and their skill. Maybe there's enough of them that I don't stand a chance or perhaps they're just plain better than I am. Sometimes it's not even a matter of skill or done purposely to grief; sometimes the enemy is just too numerous or powerful, or I simply don't have the necessary means to fight back. I've personally never been spawn trapped in this game, but I have seen it happen to others.
Rage quitting is... iffy, but it is also most certainly a real thing. Like spawn-trapping, I've been on both ends of it (again, not in this game). Basically, it's nothing more than getting so pissed off for whatever reason that you literally leave or quit the game in a rage. It's not just in multiplayer games either. In the past I've thrown controllers after getting extremely frustrated at some single player games where the AI was being "cheap". It's more common in multiplayer games, though (I remember rage-quitters galore in Left4Dead, and I've inadvertently made party members do it in some MMO's, for example).
Problem with rage-quitting, is that it really cant be confirmed as an actual rage-quit if the player doesn't have a mic or communicate otherwise. If you can't hear them curse you out or otherwise hear their delicious anger as they leave the game, you can't be positive that it was a rage-quit (anger filled or profanity laced XBL/PSN messages immediately after a disconnect do count as a rage-quit, btw). Perhaps they may have been a little annoyed when they left, but that doesn't necessarily mean that they left in a fit of anger. Perhaps they just wanted to find a calmer or more team oriented session, or got an invite, or even timed out suddenly. I know I often tend to leave immediately for an invite session as soon as my friends sign on, no matter what I'm doing.
Nevertheless, people generally almost always tend to chalk up someone leaving suddenly as making them rage-quit. The best is when they message you and claim that you rage-quit when you didn't, and your calmness and/or lack of caring about it makes them rage.