To the OP, the reason some of us are asking which model GC you have, is the GC1, GC2, and early GC3 tables used a rubber cushion that was a slightly different shape than the current cushion available. Later GC3, GC4, GC5, and GC6 all use the same modern shape. They were all called "superspeed" but were made to different sizes between the two groups. So if you have one of the older ones, and you put the modern cushions on the table, it won't play exactly right. The measurement from cushion nose to cushion nose will also not be exactly 50" x 100".
If this is your scenario, the "best" way to fix the problem is to cut the wood on the rails with a special saw set up to make the old wood rails accept the new rubber cushions properly. The two mechanics referenced above are both excellent at this exact task. Its not cheap though, especially if you have to ship the rails to someone to do the work for you. There may be other options, those two mechanics will know all of them.
<=== Not a mechanic