Already a thread for this
Hopefully the Mods can merge this one.
To answer your question the best way to Gold a Gym is.
1. Battle one down down that is full of 6 Pokemon, the higher the CP the more Gym points you get for defeating each one.
2. Stay in that Gym for as many days as possible.
3. Berry the crap out of every Pokemon in the Gym as often as you can.
4. 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.