both are again, mere corporations. While it is true they operate in more dangerous place, they can usually rely on bounty hunters among other things to safety their position. Let's not forget maintaining a battleship in operation should be a costly matter, an annoying matter, used mainly by militaristic factions - where junker makes a difference, is that they put themselves in some precary situation dealing with all these powerful pirates without in some way, showing them "hey, dealing with us is on neutral ground... otherwise taste some big guns." Plus, they are those that can pass down the big ship to the unlegitimate organisations. I can see them store a few non-operational on their guard system (apparently that base is a huge shipyard?), just waiting to be sold.
However, I dont know GMG line of ship. If they dont have a Battleship, this might be their ocasion. Logically, they are situated right next to the entrance of the outcast realm. They are openly at war against the largest drug smuggling organisation, equipped with large warships themselves. Let's not forget either, they are very near the corsair territories as well. GMG is in the worst place to be making business really. It also holds a near complete monopoly on H-Fuel, making him very rich. The time of the 80 years war are long passed, GMG has passed from rag taged miner into wealthy miner, but constantly in danger. In such situation of high danger, I can understand GMG get a civilian battleship.
Same of course can be applied to IMG, but I never felt they were as menaced as GMG.
I can accept this whole idea, as long as these battleships are less powerful than most others.