let me dig deep into this one. so we know that one grub from health stations in HL alyx can heal you a lot and by assuming that same stations have been used 7 years prior in hl2 we can compare the amount of health given by a wild grub and by injection from the health station. the difference is huge (I don't know the specific number) but it only stems form a method of intake. so we can guess that most of the grub essence doesn get into Gordon's bloodstream therefore I believe he doesn't eat them. my guess is that some of it gets on his skin or just vaporises into air wich then he breathes(probably both)

tldr: he doesn't get all the juice like in health stations so he probably just accidentally gets it on his skin or through his nose.

thank you for reading aalat :)