The idiomatic expression "gat on the act" is correctly spelled as "caught in the act." The use of "gat" instead of "caught" is a phonetic evolution of the word in certain dialects. In IPA phonetic transcription, "caught" is /kɒt/ while "gat" is represented as /ɡæt/. This difference in pronunciation can be attributed to regional variations in English language and accents. Nonetheless, both spellings convey the same meaning of being caught red-handed while committing a wrongdoing.