At the moment Theon is leading the pack for newbies name.
Didric, that could work, it comes from Theodoric so means
Theophilos is the first that comes to mind for me.
I don't know if you followed the saga of my friend and her baby Theo, but here's the short version. They named him Thierry, which is a French name pronounced tee-air-ee (or close enough). They made the mistake of calling him "theory" then changed the name to Theory for awhile and ended up legally changing it to Theodore at 2.5 years old.
The only thing of value for us to extract is Theory can be a given name for Theo. If you're up to such a thing.
Gunn is a gilsname, it means war
I know you are looking for a Theo option, but I can't get over Fabian for you.