In a general sense, one would say it's not, as TOCs like ScotRail of similar/bigger size can demonstrate that large franchises which operate different variations of services in regions hundreds of miles apart can work if managed successfully.
I can't really speak for GWR as I don't use them regularly but given the number of passengers they serve and the number of services they operate, maybe it would be time for a rethink on how it's structured in order to simplify the task at hand.
For a start, the franchise ventures too far into the territory of others. Services to Guildford, Portsmouth, Weymouth, Southampton, Salisbury, Gatwick and Brighton should be handed over to SWT, who already operate services in these areas. Perhaps that also begs the question on SWT's set up also. But in terms of GWR's, splitting the more regional services would be a decent first step.