Are you after a BB or after someone confirming it's 73x113 ?
The 73 or 68 bit is easy.As it's a GT 99% certain to be 73mm and if unsure, measure it.Also if using a shimano chainset then 110 or 113 will be about right.
Think I have a little used 73x113 or 110 somewhere if that is what you are after.PM me
Thanks