Gmail stuck on "Waiting for sync"
This question is rather ROM specific.  

In fact the custom ROM post even says it's a "New Beta from vibraturk". If you want to continue using this beta ROM, please search the linked thread or ask the creator for for more specific help on his ROM.

You might want to consider a more "established" ROM like CyanogenMod for Captivate, there are even CM9/ICS nightlies already.