"In GTA" is a phrase said usually after a "Fed post" (Anything posted online that shows conspiring or premeditated planning of criminal activity. ) and it usually means "This is a joke" to the FBI Agents watching on screen.
1st person : "I'm going to send pipe bombs to your IRL house In GTA"
2nd person : " Oh haha me too in GTA!"
FBI : "Oh alright they're just making silly jokes on the internet, leave him alone boys"
OR
1st person : "I'm boutta bomb this whole mf plane"
A griefer in GTA is really one of the most hated people in Online mode of GTA V, there is almost no benefit (If any) of those virgins that sit all day Grieving cargo, crates and even players that's minding there own business.
"lzzzzzz" "1-0" The most common phrases that adopted Griefers in GTA Online use to piss of players.