Dev: Migrate GFS to Herbie

**Describe the task**
Migrate GFS processing to use Herbie

**Acceptance Criteria**
- [ ] GFS processed using Herbie
- [ ] Has similar code coverage to the ECMWF model
- [ ] third

**Additional context**
- Add any other context about the task here.
- Or here

Start by locating the existing GFS processing entry point and the ECMWF model implementation, then review how Herbie is used for model retrieval. Compare the current GFS flow with ECMWF and identify the tests that establish code coverage. Done means GFS is processed through Herbie and has coverage comparable to ECMWF.

Written by the indexing model from the issue text.