It can mean either.
Literally it means 'Is there a car with you?'
So that can be now, at the moment, or do you own one. The context would be important.
Adding nawr (rŵan in NW) would help somewhat.
Oes car gyda chi nawr? = 'Do you have a car now?'
Although a better expression might be Have you brought the car? = Dych chi wedi dod â'r car?