with the gLCD library, are you using PHILLIPS or EPSON? If the former, then there are four options to choose from:
PHILLIPS_0
PHILLIPS_1
PHILLIPS_2
PHILLIPS_3
It is a case of trial and error as to which one produces the desired results.
If you have an EPSON display and it is mirrored, then you have come across a display I haven't seen before, in which case I would have to add a new definition into the library.