Grapefruit is a Green food. Green foods are safe to eat as a regular part of a diet.
Guideline related quotes related to Fruits-Low-Fructose-Low-Glycemic and Green throughout Geoff Bond's publications.
Eat at least six pieces (servings) of fruit per day, focusing on “Green” and “Green-Amber” fruits. ~Paleo in a Nutshell p.102
Restrict total fruits consumption per session to three servings. ~Paleo in a Nutshell p.102
Specific references to Grapefruit throughout Geoff Bond's publications.
Fruits in our ancestral homeland were much less sugary and less watery than most of the fruits we have today. Flowever, most berries (raspberries, blueberries, redcurrants, strawberries, etc.) are low-glycemic; grapefruit, apples, bananas, and many other fruits are also okay. But we have to go easy on sweet, sugary fruits like pineapple, mango, and melons. ~Paleo in a Nutshell p.48
It might be thought that this was very clear. However, in 1942, the USDA issued a new food guide that reduced the number of food groups to what they called the "Basic Seven." These were: green and yellow vegetables; oranges, tomatoes, and grapefruit; potatoes and other vegetables and fruit; milk and milk products; meat, poultry, fish, eggs, and dried peas and beans; bread, flour, and cereals; and butter and fortified margarine. ~Deadly Harvest p.45
In 1956, the "Basic Seven" groups were condensed to the "Basic Four": milk and milk products; meat, fish, poultry, eggs, dry beans, and nuts; fruits and vegetables; and grains. This time the "green and yellow vegetables" group has disappeared. "Butter and fortified margarine" has been dropped. "Oranges, tomatoes, and grapefruit" are lumped into the catch-all category "fruits and vegetables." ~Deadly Harvest p.46
What are alkali-forming foods? They are ones that have a predominance of the metallic elements potassium, sodium, iron, and calcium— chiefly fruit, salads, and non-starchy vegetables. This demands an explanation, because many of these foods, notably fruit, taste acid but are, nevertheless, alkalizing in effect. For example, grapefruit, although acid to the taste, is strongly alkalizing. ~Deadly Harvest p.109
Fruits that are both low glycemic and low sugar are "good" to eat without restriction. Fruits in this category are gooseberry and raspberry. Tomato, which is technically a fruit, is also included. There are other low-glycemic fruits, such as cherries and grapefruit, which nevertheless have a significant content of various sugars. You should go easy on these fruits if your doctor is asking you to restrict your intake of fructose or glucose. ~Deadly Harvest p.145