What about psionics? I recently saw a book on the topic at my FLGS and it looked interesting.
There is a psionic book for MongT, but alas, I don't own it. I don't feel a great urge to use psionics. (Remember, I'm the one who thinks psionics is magic and therefore fantasy, not SF.

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What other supplements are worth buying? I have the MongTrav pocket edition but that's it.
Still talking Mongoose Traveller here? Depends on what you want. I was sort of disappointed by Mercenary, but have it anyways because I like the chargen options.
1001 Patron Encounters is a good book, but for fans of CT: it's more a book of NPC encounters with a few plot seeds rather than mini adventures the way traditional patron encounters go.
From a setting-toolkit and deckplans wonkery perspective, I found Traders & Gunboats interesting.
High Guard again I liked mainly for the character options and Naval campaign ideas; not sure I'd ever do capital ship battles using their rules.
I hear good things about Agent, but don't own it yet.
Tell me more about Gateway to Destiny. Is it old or new? Is it available in hardcopy?
It's a few years old, came out shortly after T20. It was available in print, though the PDF can be had now; print might be hard to find.
Here's the PDF:
Gateway to Destiny - The Gateway Domain Campaign Sourcebook - QuikLink Interactive, Inc. | EN World PDF Store
If you look around QLI's RPGnot offerings, you'll also see a number of adventures using the setting (all the "blue border" ones).
It's set in the classic traveller setting in the Domain of Gateway in Y993 (about 100 years prior to the CT baseline.) It has one sector that is dominated by the Imperium and 3 largely outside it. It has lots of plot hooks, and is on the frontiers of two other major empires: the K'kree and the Hivers.