Be it grass, fruit, or beasts, whatever you eat, you have to live its part.

Most importantly, expect tears. The gym leader will undoubtedly cry upon defeat. As for strength, have you received any badges prior to Goldenrod? If not, you may wish to attempt at least Azalea first. If you can't defeat them, you very likely will not defeat Goldenrod. You could always request another trainer to battle you who has the badge. Perhaps they could give you an adequate assessment as to whether or not you and your Pokemon are ready.