If you want to change the operator for an airport, it would be enough to customize the scenery by changing the airline codes for the parking (or multiple parking spots at once), explained at Page 32 of the GSX manual.
If, instead, you want to create a new operator that is not already included with GSX, I suggest reading the PaintKit. However, keep in mind this information will be made obsolete soon, at least under P3D4, since the next major update for GSX, which we hope to release before the end of the month, will completely change the way custom liveries will be made, making it a lot easier.