Agree with everyone here. You could conceivably use a gerund, but the verb would be "to go" and not "to be" in order to convey a near future, i.e. "I'm going to be back at 8:30". It sounds more natural to say "I'll be back at 8:30" though.

Hi Victoria, The future with “will” is used for making a promise about an action. That’s what is happening in this context.

Julia is right, there would be too many "be", it's normally not use like that. Besides, sometimes it's not planned and a spontaneous decisions is expressed by future indefinite.

"to be" is a state verb that's not generally used in continuous. you can say "I'm going to be back at 8:30"

because the continuous form implies that an action is taking place non-stop in a specific moment. Being back happens and is completed instantaneously in a specific moment.