Already a thread for this
Hopefully the Mods can merge this one.
To answer your question the best way to Gold a Gym is.
- Battle one down down that is full of 6 Pokemon, the higher the CP the more Gym points you get for defeating each one.
- Stay in that Gym for as many days as possible.
- Berry the crap out of every Pokemon in the Gym as often as you can.
- Get straight back in the Gym if you’ve been battled out asap.
Rinse and repeat steps 1-4 until its Gold.
Best to be doing this to multiple Gyms at a time but focus your Berry unload on one in particular.