What do you think?

It's been in the OED since the 2nd edition (1989). The first citation is from 1913 "C. E. Mulford Coming of Cassidy ix. 149 Yo're gonna get a good lickin'."

It also offers a comparison with an earlier Scots spelling: "[Cf. the earlier Sc. ganna, gaunna: see Eng. Dial. Dict. s.v. Go, quots. 1806, etc.]" It seems a perfectly good transcription of how "going to" is pronounced, in US English at least, in casual conversation. The t-to-n change is an example of assimilation in phonology.

Ceci n'est pas un seing.