Asked and answered...
So I had a hunch after looking at some log messages...DMRGateway would not rewrite TG 6 as needed when TGIF was defined as DMR Network 3. When I renamed it to DMR Network 1, rewriting worked just fine.
So the XLX <--> BM works for XLX <--> TGIF also, but make sure that TGIF is DMR Network 1. You can delete the unneeded network stanzas so avoid naming conflict.