Hi there. First off, which Gimp are you using? 2.6 or 2.8?
From what I know, some kind fellow Gimp users here, told me GAP is outdated. Some people have got it to work in 2.6, and others got a headache, not getting it to work for them. Especially with the exporting to different formats department.
You know Wav. etc. And the same "frame problem" you are experiencing.
I saw somewhere on this forum, a couple of months ago, that a new GAP may be in the works in the not to distant future. I hope it's true. It would really help out alot of Gimp users. Such as myself.
I hope someone else here can confirm, if a new GAP is in the works. And help you, with a better solution to your problem.
Just note, that alot of other Gimp users have gone through this same or similar problem.
Hope that helps. Bye.