You are not altering the effect by having raw garlic with grapefruit juice. You can also have garlic with protein foods, which will also not alter its effect in your GI tract, in fact, raw/fresh garlic is better consumed with foods than without. I have a patient who eats 10 - 15 cloves of fresh raw garlic daily by adding it to home made pate'. Be sure to crush garlic, peel then consume. Crushing is better than slicing, it allows the oils to become more exposed and active, according to Dr. Benjamin Lau, the original garlic researcher from the 70's. For a better effect, swallow two to three whole cloves daily in addition to the garlic.
My book Candida Crusher has extensive garlic information in it in section 4 of chapter 7. It will be online and available as an e-book within a week or so, and is currently being formatted. It is 400+ pages and is one of the most comprehensive books on this topic made available online.