This is the schematic for the GA15RVT.
The power transformer that you purchased will Not, as your tech ascertained, work.
That one is clearly marked as for the RV series of Gibson amps.
Your tech may have gotten confused as the link you posted as his replacement is for the Output Transformer.
The Mercury transformer that you linked is a Reverb Driver Transformer.
The Hammond 272FX should work.
It may take some work to get it to fit in though.
The 5V winding can be shrink wrapped & tied off as the RVT does not use a tube rectifier.
Also, the Edcor has been suggested here: https://music-electronics-forum.com/...ad.php?t=29735