I have just brought a gumshield for boxing. I have four front teeth missing. Do I fit the gumshield with false teeth in or out to take the shape correctly?
This seems pretty straight forward. If you plan to wear your prosthetic while using the mouthguard, mold it with the prosthetic in place.
If you plan on using the mouthguard without your prosthetic, mold it without the prosthetic in place.
If you are looking for advice on whether to wear your prosthetic, or not, you should consult your dentist. Your dental appliance is not designed to resist the stress of repeated blunt force trauma. Your dentist will almost certainly suggest not wearing it with the mouthguard.