Gymprovisation is when you have to use equipment other than the standard equipment available from the gym's selection.

Gymprovisation comes in two forms:

A) a hotel or other gym with a limited selection might require you to gymprovise in one way or another.
B) you might be such a workout freak that you need to gymprovise to hit that muscle "just right".

You should be careful with option B because it can come across as douchy.