Not a test pilot
The speeds came from the manufacturer's Aircraft Flight Manual. I am unsure what a GAMA Pilot Operating Handbook is.
We experience no problem with float using a 65 knot approach as indicated in the AFM. The runway we use doesnt require a 54 knot full flap approach. The short field approach is taught at a later time as an advanced lesson, due to the nature of many of our airfields.
The guy who taught me used to say you are not a test pilot so use the configurations in the AFM. They are there for a reason. Good advice, for both me and all the 152's I have flown are all unscathed after 600 hours on type.
Kermie