Of course Gar sucks at interacting with others and actually connecting with them. He was so desperate for acknowledgment and a family ever since his first appearance, and just kept being turned down. He went to the Doom Patrol because they were "freaks" like him and Cliff/Robotman chased him off and refused to believe him when he told them about his legal guardian that later tried to kill him. And then he tried to join the Teen Titans, but again they said he needed permission from his parents so what was he supposed to do.
All the while he was feeling guilty and responsible for his parents dead and he wasn't even 15 yet. He kept making jokes and being snarky because above all he was alone and that worked as a coping mechanism! But then he was with the Teen Titans and by that point the Doom Patrol was dead and his escapism was making him unable to truly be vulnerable and open up to others. I mean, when he was spiraling bad the one person who helped him the most and told him to talk to his friends (which he, again, didn't do for a long while) was Slade Wilson. And this is important because Gar was planning to kill him, but then got coerced into a friendship with him, the kind where he felt guilty for holding Slade accountable for his actions and where he could visit Gar and hold a knife to his throat at night.
Anyways, I know “jokes” take a much uglier turn in ntt that I will not delve into, (even though I'm pretty sure it was mostly because Marv Wolfman just didn't have a problem with this type of behavior and it was the 80s), but making sexual innuendos and flirting with every girl he met did become a staple of his character even if it wasn't before. And I can't force people to like him because thanks to ntt I only do half of the time. But even with that I do believe it makes sense for him to be this bad when it comes to socializing because he's a green guy who was alone during most of his childhood and then rejected whenever he asked for help.
