So far as I have read, γαρ indicates that the clause(s) it introduces are material that strengthen that which typically comes before it. Other nuances brought out by some authors is that the material can help to clarify, explain the text it relates to, or perhaps deal with anticipated questions or objections that the author may have.
With passages such as are a bit more difficult for me to fit these in.
I guess that if I take this as anaphoric then it would be strengthening an implied "ου (δυναμαι)", with the γαρ clause indicating the reason for that inability. This would be strengthening the negation, providing the reason for it and therefore anticipating and removing the need for the question "why?"προσδραμὼν δὲ ὁ Φίλιππος ἤκουσεν αὐτοῦ ἀναγινώσκοντος Ἠσαΐαν τὸν προφήτην καὶ εἶπεν· ἆρά γε γινώσκεις ἃ ἀναγινώσκεις; 31 ὁ δὲ εἶπεν· πῶς γὰρ ἂν δυναίμην ἐὰν μή τις ⸀ὁδηγήσει με; Acts 8:30-31 (NA28)
I don't have the imagination of how this could be cataphoric here.
I think I found another example a long time back but can't find it anymore. The example was another opening word in a response to a question.