We don't have GTC here in the USA, but from what I can see, MK's does not have the gear ratios for most trannies in GTC, and so it gives U 64.xxx. Your best solution for now is to do your tranny swap, and then create an ini file for the car. Open the ini file, and edit these lines,
240=-5352 >R Gear
244=-5891 >1 Gear
248=-3817 >2 Gear
252=-2719 >3 Gear
256=-2041 >4 Gear
260=-1614 >5 Gear
264=-1201 >6 Gear
268=-1 >7 Gear
272=-2001 >Final Drive
276=-43 >Auto Gear
For instance these are the same lines from a C5R tranny
240=-3346
244=-3687
248=-2326
252=-1634
256=-1209
260=-943
264=-774
268=-64538
272=-3071
276=-43
If U enter those values U would get the stock settings for a C5R, if your using a 7 speed tranny, U will have to enter a value for line 268. Once U have entered the new values, save the new ini file, and import it back into MK's.