"Dych chi wedi mynd?"
Translation:Have you gone?
I'd stick to Dych chi wedi mynd? for "Have you gone?" (perfect tense) and Aethoch chi? for "Did you go?" (simple past tense).
Just be aware though, that where a perfect tense is normal in Welsh and British English, sometimes American English can use a simple past:
W Dych chi wedi codi eto?
BrE "Have you got up yet?"
AmE "Did you get up yet?"
Why is "have you been?" accepted sometimes but not others? I can think of only one situation in which I'd use the English phrase "have to you gone?", but in situations where in Welsh one would say "Dych chi wedi mynd i'r tarfarn?" in English I would say "have you been to the pub?" or "Did you go to the pub?", but never "have you gone to pub?" as it's like asking "Are you asleep?" You'll never get a truthful yes! (On the phone I'd ask "are you at the pub?"). I would use it in the 3rd person though.