and don't forget ox and bull!

Are you saying that ox and bull are descended from PIE *gwos? English ox is usually considered to be from PIE *uks-en- 'bull, ox'. Bull is from *bhel- 'to blow, swell'.

Or, are you questioning the semantics of the root *gwos? Historical linguists are doing two things when they reconstruct hypothetical PIE roots: first, the try for the sound of the word, and then, they try for a meaning. Etymological dictionaries use these two constructs without much commentary, but if you look at the literature, you'll see that what the roots (and meanings) are are a kind of shorthand for a bunch of data and ruminations on same. What is problematic with positing that a word for one age and sex of a cow may change over the six or thousand years between PIE and English (or one of the other IE languages)? Words change in pronunciation and meaning all the time.

Ceci n'est pas un seing.