Not exactly sure what you are asking. The marriage licence allegation transcript most definitely is on the Forest of Dean database. It can also be found in the Gloucestershire archives online catalogue:
https://gloucestershire.epexio.com/records/GDR/17/3/51/106I have found that marriages by licence usually took place within a week of the licence being issued, sometimes even on the same day. Although the allegation names the church where the marriage is expected to take place (occasionally alternatives are given), sometimes the couple actually marry elsewhere.
I have just checked the register for St Mary de Crypt - which unfortuately shows no marriages at all for September 1748.
So, perhaps they married in a different place. Perhaps they married at St Mary de Crypt and for some reason the clerk neglected to record it in the register.
Or perhaps Christopher Gardiner and Elizabeth Pool didn't marry at all, and Christopher married a different Elizabeth at some point between getting the licence and baptising his first child at Minchinhampton on 4 September 1749 (I don't know why Ancestry has indexed this baptism as 4 September 1748: the register clearly reads 1749).