Think of the GPO trumpets like real trumpets. The harder you blow, the brighter the sound. In GPO, that's the mod wheel data. A mellow sound is more easily achieved at the softer end of the spectrum. So, use lower average mod wheel levels to shape the expressive ebb and flow of your lines. Then compensate for the quieter sound by bringing the volume of the instrument up with either the volume knob in the GPO player or by activating the cc7 option and using higher cc7 levels in your trumpet tracks. If that isn't loud enough then send the trumpet(s) to a separate output in the player and raise the levels in the mixer section of your sequencer/notation program for the trumpet's mixing channel.
If you still aren't getting a mellow enough sound then use parametric EQ in your sequencer to modify the tone. Generally, a slight increase in the area centered about 300 to 400hz and a slight decrease in the area around 3khz will do the job.
Finally, the acoustic setting in which place the trumpets can have a large effect on the perceived brightness of the instruments. Choose a reverb/room simulation that de-emphasizes high frequencies (think "warm.") A somewhat wetter ratio usually makes this easier to achieve. This is consistent with the usual placement of the trumpets to the rear of the soundstage.