Is there a limit to the amount of GMP you can have at any one time?

There are two different limits, offline and online. When the game was initially released, only the offline gmp/resources were available. But further patches enabled online modes.

I've maxed out and can't gain anymore GMP (and also can't spend any of it due to research duration/limits).

Offline Limit    5,000,000 GMP
Online Limit    25,000,000 GMP
Overall Limit   30,000,000 GMP

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