Thanks for bringing up this fare, CaptainMiles!
Variations on this fare are posted from most European cities to both GIG & GRU — I saw quite good fares ex-LIS/DUB/SNN/BRU/AMS and moderately good fares out of MAD/BCN/LHR (!)/BFS/CDG/FCO/FRA/TXL/ATH/STR/HAM/EDI/CPH/ARN/GVA/ZRH .
I saw fairly good R availability on AMS-IAH-GIG // GIG-IAH-LHR, which is comfortable if not the highest mileage-earning option. Ended up booking a November trip SNN-EWR-IAH-GIG//GIG-IAH-ORD// stopover for holiday // ORD-LHR instead, priced only about €100 more than just flying home for Thanksgiving.
This is a rare excellent deal for UA travel.