Gage and his brother richard have robbed several stores. when they attempt to rob al's deli, al shoots richard in the foot. richard is caught and arrested. gage continues committing crimes, and richard is offered immunity in return for testifying against his brother. if richard accepts,

The answer to this question is his testimony will not be used against him
The Justice departement often offered up deals for their convicts which give the convicts lesser punishment if the deals will turn out to be more profitable for the departement 
In this situation, Gege is proven to be more problematic than his brother, so the deal pretty much beneficial for the departement.

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