OK, so it just so happens that I was pretty close to the limit already. The limit appears to be 5,000,000 (5 million) GMP.
I noticed I had exactly that amount and then when I went to accept some GMP as part of a completed combat deployment mission, the following message was displayed:
Unable to accept all rewards. You have reached your maximum item capacity.
Yes I know it says "item" but after closing that dialog, there are two unclaimed rewards still listed, both of which are GMP.
It seems quite a low limit as I now have 440,000 in rewards waiting to collect. Would be interesting to know if those rewards will expire or if I can keep them there until I need the money. Though I think I will just spend some cash and claim them to be safe