을게

Casual 'I'll do it' promise

Commits you to something, with the listener in mind: I'll always be by your side, I'll stand back, I'm going to take the big slice, I'll just go with this. It is a promise or a heads-up rather than a neutral prediction, which is why it only works when someone is there to hear it. 을게요 is the polite version; 을래 by contrast says what you feel like doing rather than what you undertake to do.

Register: casual, plain speech