Actions speak louder than words. Just stop whining and do it!
Literal
Actions-not-words. Noisily say without [if-ば] good [explanatory-んだよ].
ガタガタ言う = 'to grumble/whine' — ガタガタ is a 擬音語 for rattling noise, used figuratively for complaining. ~ずに = 'without doing.' やればいいんだよ = 'just do it, that's all.' Very direct and forceful.