"Good morning, do you have Gaelic?"

Translation: Madainn mhath, a bheil Gàidhlig agad?

[JohnPMChappell]

Maduinn mhath, a bheil Gàidhlig agaibh?

[Ariaflame]

Madainn. The agaibh also works. I would tend to use agaibh with someone you don't know well enough to know if they speak it, but you might use agad with a child for example.