pg_trgm index not used with commutator
You cannot.  These are different operations, and only one of them is supported by the index.  If you were to swap the order of the arguments as well as the operator itself, then it would be the same operation and the index would still support it.

Although it might be possible that you could modify the source code of pg_trgm to support this other operation.  I doubt it would be very efficient, though.