Gearset allows the selection of multiple External ID's in an Upsert operation as part of a Data Deployment.

However, the system implements an 'AND' combination across all the external ID's specified on the object.
Effectively, ALL external ID's need to be present on the target side for the match to be identified and record upserted. If all external ID's are not present, the system performs an Insert on the target org.

Please allow an 'OR' combination across multiple External ID's. Arguably, this should be the default option, but at the very least, a user needs the ability to decide how they want multiple external ID's to be interpreted.