Here's a link for Grady's catalogs;
http://www.gradywhite.com/customer/catalogs/
According to the 1978 catalog, the 205 Gulfstream, (w/140 hp, i/o) is 3000 lbs.
What year/model is your trailer? Is there a capacity plate?
I had a 1983 Gulfstream that was converted to a bracket mounted outboard. The trailer was a single axle, 1982 Loadrite, with a 4400 lb capacity. It had 16.5 inch tires! It actually did a fine job, handling the boat, but, (imo) a tandem axle is more suited for a boat of that weight.
As a matter of fact, I don't think Loadrite even makes a single axle trailer, with that capacity anymore. I think their single axle trailers top out around 3000. Not sure about other brands. Some aluminum bunk trailers have surprizingly high weight capacities.
As an example, Nextrail has a 5000 lb single axle;
http://www.nextrailofamerica.com/
Again, I would feel more comfortable with a dual axle, and the ride is smoother.
As far as coverting your single axle to a dual? I think it could be done, providing the frame is long enough, but your talking about a pretty big job, not to mention expensive.