You are quite right, it has to be "..poisto.."
and perhaps also "Tiivisteiden-" (because alone it would be tiivisteidenpoistospray).
But as to "tiiviste" is gasket the best word?
I understand gasket to made of for example rubber and used between a jar or a bottle and its lid.
Is can be of course stuck and the spray might help. (I know there exists a special oil spray for getting stuck light bulbs out, LOL)
But I think I might use "stuffing" here because I guess stuffing may be for example silicon which is a kind of glue and very difficult to remove and is used for example in bathroom if the sink is mortised (inlaid) in the "table" or worktop.
Not sure, of course, maybe the english have a more precise idea I hope.
I'm not sure which is the best word in this case, gasket or stuffing. I googled "gasket remover spray" and it turned out that it actually does exist. I also googled "stuffing remover spray" and I'm not so sure about that one... It's hard to form an opinion because I really don't know ANYTHING about this kind of sprays. Maybe we should wait until the english tell us their opinion.