Sorry, I think I answered my own question here
http://www.abim.org/certification/policies/research/requirements.aspx.
Is ABIM the only path to avoid matching again for fellowship? Assuming you are at a good university program, is this option available to all or what is likehood you'd be able to do it? Lastly, is there any way to match into IM with a "guarantee" of GI?
Yes, that's the only way to avoid having to apply and match for fellowship.
The option is available to all. But your PD will have to agree to it and work with you to make it happen. Theoretically, they could give you trouble just because they want to keep you on the clinical service schedule for your entire 3rd year, I guess. Same goes for your GI program. In your 3 research years, 80% of your time is in the lab. You'd be causing a burden on your program's clinical service. That's why it seems that you might have to extend your training for a year... to add the research year to your training instead of swapping it out.
You'd need to come up with a pretty serious research project, with a pretty serious PI... Then you'd have to convince the ABIM to let you take the Research Pathway. Since the website says that the pathway is for physicians who want to get into bench research, and
not teaching or clinical... I think they're talkin' serious research. Every single IM resident does research or a "scholarly activity" in residency anyway, so your research would have to go well above and beyond the scope of the general IM realm.
I'd think that if you could come up with such a high level research project during residency, and be willing to put the time an effort into it...
you can use that same idea, time, and effort in research during IM residency withoutthe ABIM-RP, and match into GI anyway. Its 6 in one half a dozen in the other. The ABIM gives you less worry because of the Match, but its hardcore bench research. Either option is equally challenging, and if you're going to put that much effort into research anyway, might as well do it in something that may be of interest to you (i.e. clinical)... and you'll still get into GI.
Other than that theres no way to "guarantee" a GI fellowship when you're matching for IM. In fact, you cant even guarantee that you'll have an ABIM-RP project when youre matching into IM. You should ask about it during your residency interviews. I did, and came up with A LOT of resistance (mid-tier univ. programs). A couple of programs had residents in the pathway, but most of the programs said that their residents focus on clinical medicine. I eventually stopped asking.