In Webtrends Optimize, we can ingest 3rd party data layers like GTM's, and consume this into the platform for use with Segmentation and Locations.
Consume the GTM Data Layer
Full details of how to pull the GTM data layer can be found here: Ingesting Data Layers.
Make sure you've done this.
Targeting by GTM Data Layer values
Once you have set this up, the names you see in GTM's Data Layer will match the ones you would use in our segment builder.
For example, "loyalty_status":"active" in GTM's Data Layer would translate to Data Object Attribute with a name of loyalty_status and a value of active.
Simple!