when btg back to 200$ 
you tell me! 
With this network upgrade, will it reach the December 2017 value of $ 300? Should the wallet be upgraded due to this network update?
I wont try to predict the price, hope it does!
also, if you are using BTG core wallet, yes you must upgrade this and copy the wallet's dat file over https://bitcoingold.org/downloads/ That's it!
If I hold BTG using the Ledger Nano S is there anything that needs to be done before the Network Upgrade?
Nope, just leave the BTG in the wallet on the fork date