I kept my BTC pre-fork (both) in an Electrum Wallet.
Now I transferred BTC to Ledger Nano S. Electrum is now empty.
How do I get BCH and BTG to the Nano S?
To get your BCH: Download Electron Cash (a Electrum fork for Bitcoin Cash), import your old Electrum wallet and send your BCH to your Nano S Bitcoin Gold address.
To get your BTG: the easiest way I found to get your BTG from an Electrum wallet is by following this guide[1] (requires an android device).
[1]
https://www.reddit.com/r/COINOMI/comments/7d72jm/has_anyone_succeded_in_importing_private_keys/dpzad48/