= 頑張って(下さい)!
[ganbatte (kudasai)!]
You can say this back ↓
"頑張ります!"[ganbari masu!] (I'll do my best)
"頑張る!"[ganbaru!] (casual)
as a reply or whenever you feel like it
and want to say "I'll do my best!"
before your exam, interview,
before challenging something important :)