Last summer Amazon Prime gave away the enhanced editions of Baldur's Gate I and II, but not the Siege of Dragonspear.expansion.
Can I buy Siege of Dragonspear from GoG while it is on sale and get it to work with the BG EE given away from Amazon last summer?

I ask because it says you need BG:EE to play it, and a lot of times you can't buy the expansions/dlc for games not purchased from GoG.

yes and no

1st Prime is American [doesn't work in many other countries] but that can normally be got around

2nd Prime is not a true gaming store... what they give away is normally a redeem code

if you used your prime code to unlock the base BGEE1 game on Gog then Yes you can buy or redeem SoD on Gog as well

see how to unlock prime codes on Gog watch this vid
https://www.youtube.com/watch?v=v9_UH0ZxJRE

if you used the code to unlock your game with another store like Steam or Beamdog then you will also need to get their copy of Sod

I just checked and it's in my GoG library, i thought it was an Amazon Prime game from the Prime Gaming where you put the code into the prime gaming app. It seems it was a GoG code. I should have double-checked before posting.
My bad.

cool back to gaming then

just a heads up SoD difficulty is designed for experienced well equip parties so the jump from core to insane or Bhal is hugh at first

Well, I played the original, but I haven't gotten around to actually trying EE as I was finishing another game. I've got a backlog, but was thinking of getting the expansion while it was on sale.
I'm not sure if I've played the original since 2016 according to my last save on my Win98SE/XP computer.
So do I count as experienced if I've had it since 2002? I do have a final save of the original for a C/F/M, a bard, and a F/D, but I'm unsure if I could use those because they are the 5-CD version from 20 years ago.
It might be better to replay from scratch and try out something like the Totemic Druid, which when playing the Collector's Edition of BG2 seemed like it would have been powerful in BG1, but was weak in BG2.