This is a very common issue with the GTV. Don't worry too much about it, you bag isn't going to go off, when the light is on it just means the airbag is disabled. Still, why is it happening? Two reasons:
1. Low battery power. Aibag deployment consums quite a bit of power it seems. If your battery drops below a certain level then the light will come on, make sure it's fully charged.
2. Known GTV/Spider fault. Get the Alfa computer plugged into your system, if it shows an airbag fault code then you need to have it reset. There is a workaround for this on cars with persistent airbag light problems - Tell you dealer to check their Alfa Alert bulletins and get your system fixed permanently.
Hope this helps.
Where has he been?