I was taught at school that it could also be: saya mempunyai rok. Would that also be correct?
Saya punya gaun.
Saya mempunyai gaun.
Saya memiliki gaun.
Those translations are accepted.
If your translation was rejected, then it's because you translated "dress" to "rok".
Gaun and rok is two different thing. Gaun is dress, and rok is skirt
Is 'gaunku' an acceptable answer?