Today I picked up this 1996 GT Outpost Trail, for $10.
(I am not knowledgeable on MTB’s, but thought it best to buy it first and research it later.)

It has a combination High Ten & Chromolly frame, Shimano Altus Group, and Weinmann double wall alloy rims. It is in fine condition.

My question is - What size is it?
How do I measure frame size when the seat tube sticks up an extra 2" like that?