In Hotel Gellért you can find the famous Gellért Bath and the Gellért Spa. The spa is a part of the bath complex. As tourists come to Budapest they often think of the Gellert Bath as the best in town, but in general it can be said that the Szechényi Thermal Bath is a better place to go if the goal is a nice swim in both inside and outside thermal water. Szechényi is also a bit cheaper. But, the Gellért Bath is still worth a visit and here you can enjoy a wide specter of treatments which can be booked as you arrive to the bath complex. Read more about Szechenyi vs Gellert in this article.
You just say what you want, and at the hotel they have a big crew ready to receive you within hopefully not to many minutes!
At the Gellert Bath you can also enjoy a nice massage and there are quite a lot of outdoor pools available during summer, but if you come during winter time there is in general only a little outdoor pool available, while the indoor pools are going strong throughout the entire year.
If you want to book a visit to the Gellert Bath with or without a massage, visit the following site to avoid lines and get into the pools as soon as possible after arrival.
Gellért Thermal Bath
1118 Budapest, Kelenhegyi út 4