What is the Gezera about the Merchatz the Gemara is referring to on this daf?
The Gemara in Maseches Shabbos 40a tells of the development of the Gezeirah of
not using bath houses on Shabbos.
Originally bathing in water which had been heated before Shabbos was allowed. When the bath house attendants began heating water on Shabbos and saying that it had been prepared in advance, Chazal forbade bathing in hot water completely. Entering a bath house to sweat was still permitted. Then people began going in to the bathhouses to bathe and would say that they were just going to sweat so Chazal made a Gezera to forbid sweat baths.
The Gemora in Berachos brings Rebbe's going to the bath house as proof that
Rebbi did not stop doing Melachah.
Rava rejects this proof saying that this was before the Gezeirah forbidding
sweat baths and Rabbi went in to sweat which was a permitted activity.