Soak them with Kroil or WD-40 and let them sit over night. Support the sight /barrel/upper receiver and give a few shots as described above. When I removed mine, I didn't hit 'em hard, just forceful taps and I could see them moving...I was worried about doing damage to the barrel.
Check youtube as well, there's a bunch of "how to" videos there on removing the front sight/gb.
Good luck.