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According to this February 18, 2016 blog by Canadian Wanderer Ian Glen, the sundeck is an separate, additional fee:
At both Széchenyi and Gellért men and women bath together and swimsuits – normally tiny Speedos for the men as this is Europe after all – are worn in all but a few clothing optional locations. Expect prices for an afternoon of relaxation in the baths to be around CAD$20 with locker rental but extras such as massage, admission to the outdoor naturist (read nudist) summer sundeck, or something called a chocolate treatment will all add to the overall price.