But for this progression, how does 1 conclude it's in A or Gb minor?
--2 answer my own question--am I corect to say it's all about good ear training?
--if so, I have lotsa training to do

By my own admission, I've been lazy to train my ear to the required level, I've only trained myself to hear the popular basslines or chords& not even in any particular key

It does have to do with good ear training. Even if you ears aren't that good, u can't deny the theory. If you play the bass notes by itself, you should see & hear the pattern:
Gb (6)
A (1)
Eb *passing chord
D (4)
F (b6/#5) *this is really a 3 (C#) in 1st inversion
Then, put it with the chords:
Gb / E-Ab-A-Db (6)
A / G-B-D-Gb (1)
Eb / G-Bb-Db-Gb (bass could be A also)
D / Gb-Bb-Db-E (4)
F / Ab-Db-F (b6) *really a 3 in 1st inversion
You see on that chord with the Eb, "it has bass could also be A"? That further lets me know it's a passing chord. Then, u see the last chord is a Db/C# major chord, that's why I call it a 3 in 1st inversion. It's a secondary dominant because 3 is usally minor, but here is major. And it goes right where it's supposed to, back to 6.
I would rename some of these notes, but this is what the original poster posted, LOL.
Have you ever heard a song called "Glory Glory" by New Direction? Well, when that song first starts out, it uses a progression similar to this in Eb minor.