Gateways and fleet assigned to are totally independant issues.
On joining you can chose your gateway but not your fleet. With time in the company you will be able to bid for whatever aircraft you fancy.
Airmiles. Yep, collected through airlines, hotels etc.. They are yours but some gateway seem to collect more than others. This is because some gateway are served by low costs (not many offering reward schemes) and some tend to be served by more traditional airlines (Airlines operating from LHR will tend to belong to a frequent flyer programme).