The GMAT would prefer "people WHO eat meat have more..."

Though they do use the -ing form on this test, in this case the problem is that "people eating meat have..." implies that they only have more protein in their bodies WHILE they're eating that meat, and who knows what the protein levels are when they're not actively eating meat right now.

If that's what you're trying to say, that's fine. But if the intention is to say that these people have more protein in general than people who don't eat meat or who don't eat much meat, then you need to use "people who eat meat." And this is much more likely to be the meaning of the sentence.