JA2 Gold has Jagged Alliance 2 version 1.12 and Jagged Alliance 2: Unfinished Business version 1.01. The original box came with 2 cds, one containing JA2 and one containing JA2:UB, with no content overlap.
If there are any bugfixes from JA2:UB 1.01, they have been added to the main game in patch 1.12, which is both the version of JA2 Gold and the version GOG sells.
As for extra items, the only items I know from UB that were added to JA2 are those found in the unofficial 1.13.
My suggestion would be to grab them from wherever you feel like (gog!) and then after a few playthroughs to grab the 1.13 patch to discover a whole new world. Should you need any more info about the 1.13 patch, drop me a PM.