I don't know. Normal encounters in the wild it will be 50% but the mystery gift seems to be much different. I soft reset dozens of times and never saw a single female (but lots of hidden abiliites that I stupidly didn't take!) so I don't know what the ratio is for the mystery gift. If you have a ditto though it'll be no problem, the offspring between the male Pumpkaboo and the Ditto will be back to the normal 50% male/female