[GCP] Support G4 preview instance type #3155

Review

Author: peterschmidt85

src/dstack/_internal/core/backends/gcp/models.py

@@ -92,7 +92,7 @@ class GCPBackendConfig(CoreModel):
     preview_features: Annotated[
         Optional[List[Literal["g4"]]],
         Field(
-            description=("The list of preview GCP features to enable."),
+            description=("The list of preview GCP features to enable. Supported values: `g4`"),
             max_items=1,
         ),
     ] = None