Gmail can be forced to use SSL throughout - its default is SSL for login and then HTTP but you can change this on Settings: General: Browser Connection. Although I haven't tried to access gmail through IGS ... (Interestingly, if you can get to iGoogle through IGS, you can at least check whether you have any new mail through the Gmail widget so you can go and find a more useful computer.)
I really think it may be as simple as Armynet SSL is allowed on the EGS and therefore any service that is provides gets through as that SSL is possibly not monitored.
Or just that it is webmail under Army control, with a limited degree of monitoring for abuse and traceability of account ownership, and therefore 'trusted' to a slightly higher extent than commercial offerings? Not currently having an active DII account myself, would somebody check whether you can get to RNCOM (www.rncom.mod.uk) from within the 'ring of soggy paper'?