According to your examples I think you are refering to the situation whre you ask somebody if he correctly understood something. Then:
"Get it?" would be translated to:
- "Tu as compris ?"
- "Tu as saisi ?"
- "Tu as pigé ?" (more informal)
"I get it" OR "got it" would be translated to:
- "J'ai compris" OR "Compris"
- "J'ai saisi"
- "J'ai pigé" OR "Pigé" (very informal)