Your post states "starting an MGA" and then you go on to say a "GA". Those are 2 different types of organizations in the E&S world. To be an MGA, the carrier gives you the binding authority on their behalf. If you are not already doing this kind of business, I would say it would be very difficult to break into this field by yourself. If you are writing for someone now, and doing it for your current employer, yes, you may have success in getting the appointment. To be a successful MGA, you really do need 5 or 6 different carriers, though, would you be able to do that?
If you are looking to open your own General Agency, or Broking agency, that's a bit easier....but again, you would need to be able to trade on existing good relationships for an apointment.
The bigger issue for either an MGA or GA is this question - how much premium can you feed your markets? I am seeing our contract binding carriers (Century Surety, Nautilus, General Star, Penn Star) cancelling existing appointments for lack of production. To convince someone to open up a new shop, you are going to have promise premium volume, and then deliver it.