I have these windows on my 1964 Islander, and recently had to replace the sealant between the ends of the gasket, as there is quite a large gap. (1/8 inch or so) and it was leaking. There was sealant in the gap before, but it was no good any more.
This could be due to the gaskets being very old, and you may not need sealant for new ones as others have mentioned, but I use it on the old ones.
BTW, I have not seen any sealant on the rest of the gasket, just at the joint of the two ends.
1964 Islander 32
Saint Helens, OR