Originally there only used to be Window#show, the C++ function that runs the game's main loop, and which can be safely ignored when looking at a profiler.
However, due to the way exceptions have to be handled and translated across C++ and Ruby, Gosu renames the C++ function to show_internal and defines a new method called show with additional exception handling:
https://github.com/jlnr/gosu/blob/master/lib/gosu/swig_patches.rb

tl;dr it's just Window#show in disguise. :)