Is it an actual gasket or is it just butyl rubber put down between the fixed light and the frame? We installed opening ports where we had fixed ports it was more like caulk. Different acrylics require different sealants. Butyl rubber is most often grey but some port light replacements come with a thick black rope type rubber to use as a sealant (I don't recommend it as it was very hard to use). Try this thread, Pearson 30 Port Lights and this thread as well which has a really useful link to CompassMarineButyl Tape
Thanks KDH. Our ports (both fixed and opening) are glass, and have a rubber seal around them, so I think I'll follow the advice in one of the threads you sent the link to-using butyl between the frame and fiberglass, and Dow Corning 795 between the glass and frame.