The F25 manual transmission was fitted to many vehicles in the European Opel production line up including the Opel Calibra. It was a 5 speed transmission with the following configuration:
There have been many references to the numbering designation that Opel elected to utilse for gearboxes, one of the more common arguments is that it refers to the ft·lbf torque capacity of the gearbox, in this case being a 250 lb·ft (339 N·m) torque limit. Other arguments include that it is simply related to the engine size (the F25 gearbox was often fitted to 2.5L engine vehicles).
This argument, that is engine size is the justification for the numbering could be disputed as the Opel Vectra B with the 2.6L V6 engine was in fact fitted with an F23 transmission.
As an example the F25 transmission was fitted to the Opel Calibra 2.5L models (C25XE and X25XE engined models)
The numerical designations usually indicate the maximum torque rating of the gearbox in Newton Metres. For example the F40 gearbox is rated at 400Mn or 295ft/lbs torque. The M32 gearbox is rated at 320Nm. The F17 gearbox is rated at 170 Nm, and so on.
Link to reliable source of GM gearbox specifications.