They released a conversion kit for the pre-2007 GT's to put a 27" in (the normal size was the 25") for a PGA Tour Golf conversion. They had you move the lower monitor mounting bracket for the larger size to fit in properly. See instructions below.
If yours was originally a 25" converted to a 27", the original holes may still be there to move the bracket back for a 25" replacement. If not, you can do the reverse of the instructions below (remove the current monitor first) to fit it for a 25". You'll need a 25" monitor bezel afterwards...