"ĝoji" according to the dictionary, is a verb (tho commonly one does derive the adjective - "ĝoja", feeling, showing or causing joy) is, I translate, "to feel in the heart lively pleasure, ordinarily for the possesion of real or imagined good"
"feliĉa" by the same dictionary is, of a person, "feeling spiritual contentment" or "favored by fate"
to muddy the waters further, there's also 'gaja' which is "being in a lively, easy-spirited and well-humored state, and showing it through external signs, gestures, etc"
it's nice to have enough happiness, that one needs several words to distinguish its qualities.
estas feliĉa, ke oni havas sufiĉe da ĝojo, ke oni bezonas plurajn vortojn por distingigi ĝiajn ecojn.