gYu thog is pronounced "Yoo-thog." mgon po = "gön po" The capital letter indicates that Y is the root letter, not a subscribed letter---i.e. གཡུ་་rather than གྱུ་.
Technically Wylie doesn't use capital letters, but it looks like your book has altered Wylie's system either to indicate all root letters, or only those occurring in personal names. Wylie transliteration is not a no-brainer. There are many silent letters, consonant clusters which radically alter their sound, etc. To pronounce it correctly you first have to learn how the Tibetan script works.