Theo, the best manual I know on repairing databases with gfix is

http://www.ibphoenix.com/ibp_db_corr.html

Recalling your previous post I should say that if database was
corrupted because your customer's computer run out of disk space,
there nearly are not chances to repair it, perhaps IBPhoenix can do
this but not for free.

Good luck, Alexander V.Nevsky.