Got your PM, I don't actually know these cars in depth, just know about them and there common problems, the alternator for example is a common problem so would explain that.
Did the problems only start after the remap and modifications?
If your car is out of warrenty then I would not worry about letting them know about the remap, the last thing you want is for them to overwrite it, tbh as said it is best to take it to someone who knows, with a error code unless you can narrow it down to a specific component then you will just waste time and money, replacing things etc and chasing your tail trying to suss the problem yourself. It sounds as if you may have multiple problems by the /starting/revving problem as well!
Is the battery in good condition and associated connections as said as this can play havok and cause similar problems with modern cars.
The GEM module would explain it, as the name suggests (Generic Electronics Module) this manages the electronics so would explain why you have various faults but you need to be sure it's this before contemplating replacement as if it's not you will be going to a lot of unneeded expense!
Martin