"imagine that the house of representatives and the senate have passed different versions of a bill regarding gun control legislation. since the bill passed by the house of representatives is different than the version passed by the senate, what is the next step for the bill to become law? "

I believe the answer is: The bill will be sent to a conference committee.

The conference committee has the power to Reconcile the differences that Arised due to the differences in legislations created by both chambers.
The line up of this committee is made up  of House & Senate conferees and only held the position temporarily.