Spindle diameter on the FSA Gossamer TT MegaExo crankset
I believe Gossamer is more of a ‘level’ of FSA products rather than a particular crank model. 

MegaExo is an FSS external bearing bottom bracket type for BSA threaded shells. It uses a 24mm spindle diameter. 

According to this article  24mm spindle external bearing road cranks are compatible with BB90.