Missed approach is a deliberate and formal part of a procedure - typically flown from DH at a point defined on an instrument procedure when the MAP is reached and the conditions to continue to land aren't met, or earlier if the aircraft has drifted too far out of limits on the approach.
GA is a response to any number of incidents but generally you can see the runway, but have for whatever reason decided that a landing isn't viable. It is normally into a visual circuit, whereas the MA will have been into a published missed approach procedure.
There are obvious similarities (not landing!), but they are quite different. The first and most obvious difference being that in a MA you probably can't see the runway, and in a GA you almost certainly can.
G