I have completely change my views on cooking Roasts do to Members of this site educating me and showing me results with their cooks and learning more of the science of how meat cooks by my friends in Australia.
This is some conjecture on my part, but it appears the closer to the finished temperature you can cook the meat, the more even the muscle will cook and the tender/juicier it will be. Now, we cannot cook a Roast at 135°, so I think we need to be in that 200-225° range. I am also advised by some, to sear/brown the meat 1st as if done in the end it can put tension on the meat fibers and lessen the quality of the meat.
I do not think the GMG will be an issue. Whether you buy into the sear/brown 1st is on you. I am not doing beef this year, 1st time in a couple of decades so I unfortunately offer you theory! I think very sound theory.
I think you said 4.9lbs? I think I would go 225° with very accurate probe in dead center of roast, and plan 4 hours. I am told that a great benefit to this low cooking is holding in a pit for an hour or so with no harm. The DB can be set at 170° if it gets done early...I am also told cooked at these low temperatures, a rest is not needed as the muscle fibers do not need to relax...
I wish I could be more helpful...and with the size of this 1st one, I would not worry about trussing it if you have not done it yet!
Just know it is going to be one fine meal!
You really do need to know what the GMG is running at grate temperature though...As well as IT and you said Rare...125°, not 135°!