I have also had problems with Gmail on MTN in the last two weeks both on my PC (firefox 4) and on my phone (GMail symbian app). This leads me to believe it is a problem with MTN considering other people are not having trouble with Gmail.
On my PC it refuses to log (i.e. loads forever) but it usually works after several attempts. My chat in Gmail also periodically loses its connection.
I check Gmail on my phone with MTN and it also takes a few attempts each day before it loads, on full signal HSDPA. There's definitely something up with Gmail and MTN. My Gmail on CellC on my pc is almost instant since about 2 days ago.