aqwsed12345 :
Thank you for your exhaustive approach to this question. While the expression "God Jesus Christ" doesn't appear anywhere in scripture, the expression "Lord Jesus Christ" does.
The relevant scriptures are 1 & 2 Thessalonians 1:1. These read "... to the congregation of the Thessalonians in union with God the Father [Θεῷ Πατρὶ] and the Lord Jesus Christ [Κυρίῳ Ἰησοῦ Χριστῷ]". I note that this is grammatically identical to the expression "God Jesus Christ [Θεῷ Ἰησοῦ Χριστῷ]" in the inscription, and so agree I confused the genitive and dative of "Jesus" which can be identical.
However, this does leave us with this rather perplexing inscription which reflects a high christology that didn't exist until the fourth century. There are no other inscriptions with this wording, most use the scriptural expression "Lord Jesus Christ", not "God Jesus Christ". It could be that this small group believed in a form of modalism, like Monarchianism or Sabellianism, which taught that God and Jesus are not distinct. The more I think about the wording of the inscription, the more likely that seems. Of course, today they would be described as heretics but at that time Christianity was more fluid.