I never got a chance to get the gold Ace Training medal because we can’t increase the prestige of gyms anymore !
I don’t think you can ever get that one again…
Unfortunately you wont’ be able to make further progress on it. It will also remain on your medal page.
You don’t receive now, because the new system give you a badge about the gym you help or defeat.
And if you give berrys of another pokémon from you team, you increase you badge since basic status into gold badge, receiving XP to update your badge.
Hope help you.
From Niantic Support: “Ace Medal: This achievement is also retiring. You’ll still keep the Medal but, since you can no longer train at Gyms, you won’t be able to make further progress on it.”