GPSMapEdit is a 32-bit application, which uses up to 2GB of memory. On 64-bit system you can increase this value to 4GB by patching exe header with editbin, but I doubt if this will help with your file.
When GPSMapEdit opens an img container, it shows its content as a list. You don't have to load all at once, you can select only some elements to load and then eventually repeat the same process with other elements.