I'm going to try not to read anything into the politics on this one. I can see how this could be off-putting - to be talked to about a vacancy and then be told you can't have it is going to weaken your trust in the organization and that's fair.
Steps you might take...
1 - Ask for more information
It is totally legitimate to point out the inaccuracies of what you've been told and to retell the history of this to your boss or another trusted supervisor and say "I'm confused, what's going on here?"
It's also totally fine to ask for corroboration on why the policy exists and how likely it is that you'll really move to where you want to go - have other people done it? How likely is it? What behavior do you have to demonstrate to make the transition more likely or is it virtually guaranteed? Why should you believe it? These are all fine questions to give to the people offering you the jobs. You can even voice the fear that if you become a great customer-collector, they will keep you in GB instead of moving you to HR.
Also - voice your interest in HR. Be ready to explain why you prefer it, your strengths relating to it, and why it makes more sense for you personally. Stay away from any negative talk about GB - you never know the full history of the listener and negative talk can easily be taken the wrong way.
2 - Take or leave the offer given
Once you have the best information you can gather and you're certain that it's GB or nothing - you need to choose based on that. As someone else said - 2 years is a long time, a lot can change. If you don't like the work you will be doing this year at the organization, you need to look for other options outside of the organization.
You can hedge a bit, if you wish - and delay giving an answer while you look at the job market for a short time, but you do owe your current employer the decency of giving an answer in a timely fashion. There is no shame in saying "I'm sorry, but GB isn't what I want" - as long as you realize this may be the only offer. It's pretty hard to tell whether they would change the policy to keep you around, or whether this is something so important to them that they will let you go.
If, conversely, you think there is interesting work in GB and that the knowledge you'll gain is worthwhile to future work - then go for it.
3 - What about asking for promises?
It's a very situational thing on how well this would be taken - but my experience has been that no matter how formal and written a promise is, they can't guarantee your future promotion and to make you believe otherwise would be dishonest. Just about any promotion has two factors - your current job performance and the state of the business. So if you don't do well in GB or the business downsizes and doesn't need more people in HR, you will not get a promotion to HR - regardless of any letter they sign.
4 - How about later?
Among your questions it sounds like you are also wondering - if you take the job in GB, and become good enough to earn a promotion - what are your options about moving back to HR at that point? This is very much an "it depends". It has to do with how promotions work in your organization, how departments do their hiring and recruitment, and how your rejection of a promotion is interpreted by the boss.
Usually when a transfer comes up - the employee and the two organizations know about it and collaborate in some way. In a big company, there may be a transfer process, or it may be as informal as a personal talk with your supervisor. But a transfer is generally quite different from a "promotion" - it's getting someone a new job, rather than saying they are awesome at the current job and should have more responsibility.
The day your boss comes in with a promotion in hand for you is NOT the day to surprise him with your desire to leave his group. Nobody really loves to hear that their employees don't want to work for them - and the day a boss has gone to bat to get someone a promotion is not the day he wants to hear, "I don't want to be here"
Transfer discussions are something that should come up as part of your yearly reviews when you talk about your personal goals. In a perfect world, you'd have a boss who helped you structure your work in GB to make the transfer happen as smoothly as possible.