Final answer:
The correct punctuation mark to use in the given sentence is a comma.
Step-by-step explanation:
The correct punctuation mark to use in the given sentence is a comma. The sentence 'Don't look back, someone may be gaining on you!' is a compound sentence with two independent clauses, 'Don't look back' and 'someone may be gaining on you'. The comma is used to separate these two clauses and show the relationship between them.