Answer:
Your answer would be that the sentence above is a simple sentence.
Step-by-step explanation:
A simple sentence is a sentence that consists of just one independent clause. It does not contain any dependent sentences. An dependent clause, unlike an independent one, can't stand alone as a sentence because it does not represent a complete idea. It depends on the main sentence to do so.
As you can see, the sentence above is a made up of an NP subject (everyone) and a VP predicate (rushed into the gym). Both the subject and the predicate make up the independent clause. The sentence does not depend on any other element to complete its meaning.