I love Gamzee's fuckery. The moment where he almost casually, with a quiet, affable satisfaction, puts Vriska and Tavros corpses into the same kernelsprite, is one of my favourite moments from the entire comic. It's so magnificiently, bluntly, ironic-hell-like evil. And you need to have read several thousand pages to understand why.
But I still like it a lot when he gets assault-rifle'd for three pages.
Vriska is more complicated but similiar. She is actually far more consistent than Gamzee, who alternatingly acts for the lulz, for his dark masters, out of supposed insanity, because he's a rap clown etc etc. Vriska does what she does because she is that child beating a frying pan with a big metal spoon screaming "Look at me!". She was an insuffferable bitch with special snowflake powers, but it was very consistently potrayed how her behaviour had put her into a pariah position, and how it eventually killed her even though her best friend tried, in the end, very hard to keep her alive. I think Vriska's arc was worth it. The only real problems I have is
1. with how much of the plot she dominated (How much of the troll drama would've happened, could've happened if young Terezi had stabbed Vriska a sweep earlier? Pretty much only Gamzee randomly getting off the juice and into violent murder mode, and the whole Eridan debacle that went nowhere[but removed some useless trolls from the story, so, worth it]) .
2. with how much time and life she sucked out of John. Not that John was ever very proactive, but he could've had some growth on his own(... which he only now sorta got). Instead he got yanked around by a metaphorical leash for all of Act 5. Their logs were still entertaining, but nowhere as good or interesting as Dave and Terezi's.