That is a 2-5-1 in F.
The tritone sub is used on the dominant (five) chord.
G/F A Bb D 2 chord
Gb/E Bb C E 5 chord (this is the tritone sub--Gb has been Subbed for C)
F/E A C 1 chord
If this was in C, the G chord would be a dominant 7th chord instead of a minor 7th.
But the chord that I'm substituting is the 1 dominant 7th, not the 5 minor 7th
LH/RH
A/GBCE
G/FABbD <------- I'm not substituting this key
C/GBbCE <------ I'm substituting this key, and this is a dominant 7th chord when play in C.
F/EAC
2-5-1 in F is basically a 5-1-4 in C. I guess the best way to see it is by playing it in song.
Do you know that song by Christ of the Nation called "when you think about the Lord".
It is using that progression, it is basically 1-7-3-6-5-1-4 in the beginning part of the song.
Or the first part of that old christmast song "we are the reason that he gave his life".
It is using that progression too.
It is usually used for slow type song.
I don't think those 2 songs played in C, but the progression 5-1-4 is being used there.
Try it and let me know what you think.