Dorico has no definition for a trumpet in G. While rare, such instruments DO exist.
The ‘normal’ trumpet is available from A to F, and the piccolo in A and Bb only.
(and yes, I could get sorta-there with a Horn in G, but that’s the wrong octave)
If you have another notation program were you can customize instruments, try making a trumpet in G there and exporting it as MusicXML. Dorico will probably create a new instrument when you import it.