How do you meaasure a GT frame.

[SeeingisBelievingGTMuseum]

With a measuring tape, center of BB to center of the top tube where it meets the seat tube.

 

[Rampage]

My local scrap metal dealer measures in kg's!

 

[Barneyballbags]

Just to add to the confusion (and I know it's not relevant to the OPs bike), I've got a STS frame which is an 18" in GT sizing and which measures 20.5" centre to top and 16.5" centre to centre :?