|amenity = gym|
|Using this tag is discouraged, use leisure=fitness_centre instead.|
|Used on these elements|
|Tools for this tag|
This feature has been labeled as deprecated. The recommended replacement is: leisure=fitness_centre.|
The reason is documented in Deprecated features. You are still free to continue to use or interpret this tag as you see fit since OpenStreetMap does not have “banned features”. Under no circumstances should you (semi-)automatically change “deprecated” tags to something else in the database on a large scale without conforming to the Automated Edits code of conduct. Each of these changes would be reverted.
1: leisure not amenity
Most people seem to agree that 'leisure' would be better than 'amenity' as the key to use.
See Gym / Fitness centre#leisure not amenity for details.
2: Ambiguity of the word "gym"
"Gym" has two meanings which are quite different (though from the same origins). Native english speakers are most likely thinking of a Gym / Fitness centre when they use the word, but others often get it confused with Gymnasium. From this ambiguity we might conclude that using the word gym in a tag is a bad idea.
See Gym / Fitness centre#Ambiguity of the word gym. Details of this problem have been moved to there.
Which tag to use?
If you are mapping a gym as in fitness centre see Gym / Fitness centre, where various options are presented (unfortunately the "correct" tag to use has not been widely agreed upon yet).
If you are mapping a gymnasium see Gymnasium.