Well I could see it being "seized" to the block a bit.
Previous owner might have gunked up the gasket really good too.
Thinking if it were me I would just hit the part the pulley attaches to with a hammer.
Tap the top, tap the sides, if you can get in there tap the bottom, you get the idea. LOL.
Just need to free it up a bit.
IMO do not use the supplied paper gasket that came with the water pump.
I used a FelPro when I did mine.
Also no gunk on the FelPro. Was curious about this.
On Felpro site they state very clearly not to.