Seriously; genuinely; used colloquially as an emphatic intensifier to indicate earnestness or strong feeling.
来月の試験はガチで勉強するつもりだ。
I plan to study seriously for next month's exam.
彼、ガチで怒ってるから今は話しかけないほうがいいよ。
He's really angry, so it's better not to talk to him right now.
そのプロジェクトにはガチで取り組んでいます。
We're tackling that project in earnest.