Guessing you installed it with sealant? Probably just a stuck gasket, a moderate rap to the side of the gov hsg should break it loose. A wooded rod would be a good transfer device. I like a slow moving heavy hammer, not a fast light one. Easier to sneak up on success.
Tired as can be and up against a time wall - time to step back and take a breath. I communicated with my friend Red Hamilton as well and the thought is consistent that it is bonded. He suggested a putty knife might be helpful.
I tapped it with a plastic mallet earlier but they were light because I wanted to think about it over dinner before taking it to the next level. I've got a couple of hand sledge hammers and a heavy dowel out there so its time to saddle up and go back to the airport. ARGH!
It might take a putty knife, an aggressive nudge with a mallet, and maybe some heat. If there is sealant on there it's been cooked on really good. Try the mallet with a wood dowel side to side, or at various angles.
It's almost 2 in the morning and I just got back from the airport I did use my little sledge hammer and a thick wooden dowel (~1.5" dia.). After a few high mass low velocity whacks it popped loose that is the first time since new that it has been off (~14 years). I re-torqued the prop bolts (42 ft-lbs) and safety wired them. Torqued the governor adapter cover nuts (204 In-lbs). It is ready to test and check for function and leaks then close up and fly to Jasper, TX for a race on Saturday.