sophiemccallum
Hi, this is actually already tagged as being the Pure Gym. The 'fitness centre' tag does not render well on to the map in this legend but you can see from the wiki (http://wiki.openstreetmap.org/wiki/Tag:leisure%3Dfitness_centre) that it is the most appropriate in this case. 

You should be able to view these tags in an OSM editor such as JOSM or by using the query (question mark symbol) or display map data function (the layers symbol) on the right hand side of the website.

Thanks! 