TGIF is much older than you might suspect, dating back to pre-Roman days.
it originally was

Tempus gratias in fugit

which is a variation of the simpler 'Tempus fugit' (Time flies).
TGIF, however, was normally spoken in exasperation, after long periods of
tedious and boring labor.

Loosely translated, TGIF means 'by the grace of Time in passing', or even
more loosely, "Thank God we don't have to do THIS any more!". One might even
suppose it could be interpreted as "Can you imagine doing THIS the rest of
your life!!".

All translations would, however, have in common the theme of 'gratias' or
thankfulness/grace at the ability/condition of time passing/not-staying-still,
i.e., we are glad that time passes ... we would be unhappy if time were
stagnant (and we had to do this crap forever).