In general, most of the parts are different. GMC parts seem to be a little higher-priced and a little harder to find than the same parts for a Chevy truck engine.
The GMC 302 is an excellent engine that is more easily adapted into GMC trucks than into Chevrolet trucks (the GMC engines are a little longer than 216/235/261 Chevrolet engines.
Maybe others will state an opinion on the value of an unknown-condition 302 (or, any used 40-50 year old engine). Shipping cost is also a consideration when setting a value of an engine (from a buyer's perspective).
Here is a Convert to a 302 Tech Tip
found elsewhere in this forum.