Can't compare to the other books you mention because I haven't got 'em, but for what it's worth...
"So, what does the Gateway PDF have in it?"
- 9 page general history of the domain up to 995, followed by 4 page unified timeline
- 14 pages covering 5 human subraces, 15 non-human races, and 4 uplifted races. I think (in my newbie way) that maybe half of these are not well-known.
- 31 pages on regional powers: imperium, the K'kree/Hiver/Solomani powers, pocket empires of assorted size, megacorps, and certain powerful individuals. Lots of points of tension and potential conflict/adventure here.
- 79 pages of sector data, with 16 quadrant maps and complete set of UWP++ data, plus descriptive text entries (landgrabs) for about 3 systems per quadrant.
- 15 pages on "Life and Adventuring", i.e what the place is like and some tips/seeds.
- 31 page adventure called Cold Fuzion which seems generally decent.
- Appendices collating (repeating) some of the UWP++ stuff.
- No artwork (yet), those pages are all meat.
"How d20-specific is it?"
Almost entirely non-specific. Ability modifiers for races, stats for some NPCs in the adventure, I can't imagine any issues.
"what good plot hooks are there?"
It's not quite "76 adventures", but there's a pretty good supply to be found and some explicit seeds. As many are campaign ideas as adventure ideas -- it's a big scope, high level look at the domain.
Also, the QLI "Kursis Charter" and "Objects of the Mind" adventures are set in it, so you could work them into a campaign.
"What does it add to the Traveller background? What's unique about it?"
Can't help you there. I didn't play Traveller between 1982 and 2003.
It's generally well written and well laid out, good professional RPG book design -- better than the LBBs or the T5 drafts. I would have liked a CSV file with all the world data.