This way of pronouncing "го" not always is the case.
Definitely, the endings "-ого", "-его" of adjectives (for example, хорошего, доброго, злого) are pronounced "-ова", "-ева", as well as pronouns (этого, того, его, твоего, своего, ничего).
However, when "го" is not part of the ending it's pronounced "go" ("ga"): город [górad], горбиться [górbitsa], подмогой [padmógaj], дорогого [daragóva].