...but my first GUI was Rogue.
What, no love for castle.exe?
Rogue is from 1980, so it's at least 4 years old than Castle Adventure and the Tandy 1000, both of which came out in 1984.
Rogue is actually older than the PC; it was ported to the PC from PDP 11 Unix 7th Edition. It's actually a bit older than 1980, but that's the date that wikipedia gives.