I had some trouble finding the part of the "song" you asked about. I assume it is this part starting at 4:59:
The tonality is G minor (I suppose G aeolian if you want to get picky.)
...what function does each chord serve?
If we are wearing our music theory hats, function means identifying the chords within a key and the harmonic roles of tonic, pre-dominant, or dominant.
This is how I labelled it...
Gm |Gb Bb |Eb dm cm Bb |F |
Gm: i |? III |VI v iv III|VII |
Right off the bat we see there isn't a dominant function. There isn't an actual
viio. That isn't unusual in rock music. But it lets us know we are using a different playbook than that of classical harmony.
Of course the question is about the
Gb major chord.
Technically it's non-functional, because it isn't a chord from the key or a closely related key.
As @PatMuchmore points out
Gb is a chromatic mediant of
Bb. You might also say it's a chromatic passing chord, because of the step-wise motion connecting the chords. Although the bass moving by roots sort of undermines the idea of a passing chord. Also, the chord is harmonically important so calling it 'passing' isn't really appropriate.
If Roman numeral symbols is the concern, there isn't one to give it. Either skip it, or just write
Gb or 'chromatic mediant` under it.
It's worth noting that while many times a chromatic mediant chord is often a borrowed chord (like
Eb in C major where the
Eb is borrowed from C minor) in this case the
Gb is not a borrowed chord.
Gb could be respelled
F# in which case it becomes a non-functional chord built on the leading tone. In classical harmony that leading tone chord would be a diminished chord and strongly indicate the key. But that doesn't happen here. So while these chords on the whole are diatonic the progression eschews basic classical harmony.
Of course, calling
Gb non-functional should not be misconstrued to mean 'bad', 'wrong', 'weak', 'unimportant', etc. This chord creates the whole psychedelic feel of the progression. Without it the progression would be plain diatonic.