If you want to indicate the octave shift with a clef, and not have it ‘assumed’ in the instrument; then you’ll need to create a new guitar instrument in the Instrument Editor that doesn’t transpose, and which uses the relevant ottava clef by default. (You may also need to set the Notation Options for clefs to respect the octave.)
See this similar post about Tenors. Dorico already comes with two Tenor “instruments”: one that assumes the octave shift, and one that doesn’t. I suppose there’s an argument that it ought to come with similar instruments for guitar. But it’s easy enough to make your own.