Get a dead blow hammer and give it a rap or two on the end.
If that doesn't work, see if you cangently pry one end of it up. Be careful where you pry, you don't want to break it.
If that doesn't work, see if you can get a gasket scraper between the head and manifold.
Work the scraper down the manifold to help break the gasket.
I had to do a little bit of prying to get mine to break loose. A little bit in the back, and a little up front did the trick.
Strongly depends what the PO used to “seal” the intake manifold.
As mentioned gently(!) prying can does the trick. I used the neck there the thermostat housing attaches. Not the housing itself. Putting something in between the manifold and the heads/block is the last resort. Be careful not to scratch anything or you have to go to a machine shop later.
I gather at this stage you have tried a number of addresses. Hard to what the person did that put it on but that’s what makes working on these cars “fun”. I have a stout putty knife in my box for these types of problems. What works for me is taking it slowing hammering it in at a number of spots around the intake an inch deep to break the seal.