Review the original item:
Gustavo can tolerate any lecture, no matter how boring the instructor
is,
in his mind, Gustavo imagines that he is on board the starship
Enterprise, blasting Romulans from his post on the bridge.
You chose the correct way to fix the error:
Gustavo can tolerate any lecture, no matter how boring the instructor
is;
in his mind, Gustavo imagines that he is on board the starship
Enterprise, blasting Romulans from his post on the bridge.
Gustavo can tolerate any lecture is the first main clause. In his mind, Gustavo imagines ... is the second main clause.
You are always correct when you use a semicolon between two related main clauses that create a comma splice.