GRIP was put into "maintenance mode" (in other words, no new features of NX added to GRIP) before the face selection rules were added to NX. There is no provision in the GRIP language to use the face rules.
If you switch from GRIP to the NXOpen API, you can use the face rules. You can record, modify, and run journals with the NX license(s) that you already have. If you acquire an "author" license, it will allow you to compile your code, which allows you to do a few things not available with journaling alone.
To start, I'd suggest recording a journal while using the face rule that you are interested in. Study the resulting code and see if you can modify it to your needs.