I can't thank you all enough for this. Similar fare if originating in London. If you travel to London and Rio this is a huge money saver and the GPU eligible flights are a bonus.
As for stopovers, I was even able to go London to IAH (then stopover for 2 weeks), then IAH-GIG-IAH (then stopover for a month), then back to London.
Looking at booking more but you can only upgrade with one GPU for whatever is all in about one day's time. In other words, if you are stopping over as long as I am then you lose the ability to use only one GPU from LHR-IAH-GIG but I wouldn't want to do all that in a day anyway and for the savings it's worth it.
Thank you! It's almost impossible to find an affordable ticket to Rio that's upgradeable. W fares are usually over $2K.