This is another rant about hideous Proto-Indo-European roots still reconstructed in the 21st century that should have been dumped in the 1960s along with Woodstock. I love Proto-Indo-European (PIE) and am fascinated by it but I also hate unjustified reconstructions and "junk linguistics". On that subject, let's now talk about (*)*ǵalak- or similar forms designated as the PIE word for 'milk' in addition to the more substantiated root *melǵ-. I don't think a person truly understands PIE until they recognize the myriad of shoddy reconstructions out there in its name that need to be dismissed.
We have Greek γάλα (gen. γάλακτος) & γλάγος 'milk' along with Latin lac 'milk' (gen. lactis). Based primarily on this, Douglas and Adams have reconstructed *ǵl̥lákt- , have then attempted to add dubious cognates from Indo-Iranian loaded with assumptions, and have concluded (or merely asserted without firm basis rather) that "[...] both the archaic morphological shape and the geographical distribution would seem to guarantee this item as at least a regional word in PIE."
In my view, a more reasonable, alternative view is that the Greek and subsequent Latin forms are from Hittite kalaktar meaning more generally 'nutriment' and have nothing to do with PIE at all. This would be one of those Greco-Anatolian Wanderworts which spread during the 2nd millennium BCE along μέλι 'honey' which I've just talked about before. Whether directly or through an intermediary, this must be where internal -kt- comes from while word-final -r has been deleted in the Greek loan. The Latin form must then be from Greek. The word even finds its way into Egyptian as ỉrṯ.t 'milk' (*yarāṯat /jəˈɾɑ:cəʔ/). We know that the word must be from Hittite or similar Anatolian dialect because it can be further derived from the native verb root kala(n)k- 'to soothe, satiate, satisfy'. Reconstructing a protolanguage root that's unanalysable despite an etymology already available with a clear historical source is the kind of sloppy, unacademic nonsense I loathe with a passion.
 Douglas/Adams, Encyclopedia of Indo-European culture (1997), p.381 (see link).
 Puhvel, Hittite Etymological Dictionary: Words beginning with K (1997), p.19: "kal(l)aktar, galaktar (n.) 'soothing substance, balm, nutriment'" (see link).
 Woodard, The Ancient Languages of Mesopotamia, Egypt and Aksum (2008), p.181 (see link).