I want to book LHR-ORD-CVG / CVG-IAH / IAH-LHR and found flights with R availability on both TATL segments.

Do only the INTL segments need to be in "W" to use a GPU? Can I book the domestic segments in "T" or whatever the lowest available bucket is? Would the ORD-CVG segment still get upgraded the GPU if it were in a lower fare bucket since it is domestic?