going to

[AirbusA321]

It it OK or poor style to say "goynto" or "goyngto" instead of "go-ing to"?
I've heard that pronunciation a few times on TV.

 

[Rover_KE]

It's commonly heard in rapid informal conversation.

 

[Skrej]

Often it's reduced even further to 'gunna' or 'gonna'.

This is a natural aspect of spoken English, but you should never write it. Unless maybe you happen to be writing dialogue.

Edit: Here's one example of 'gonna' in this song.