When feeding via g-tube for medical reasons, typically the person being fed does NOT eat orally. My 11 year old daughter is fed 100% of her calories via g-tube, but she does drink water every day. It is unlikely she will ever have cavities because of the absence of food in her mouth.
However, we still go to the dentist at LEAST once a year and we brush her teeth daily. She hates it, but we do it. Oral health is important to overall health and the evidence of that seems to increase regularly.
We use an organic toothpaste, a teeny tiny very soft toothbrush and we do not brush very long. We count to 20 slowly for the upper teeth and another 20 for the lower teeth as we brush. (That way she knows the end is coming.) We do not floss as there is not food between her teeth. Her teeth are white and shiny and it makes for a beautiful smile!