Added to original post: [Update B June 23 2018] bitcoingold.org is back up.
   Another test pool https://test-pool.bitcoingold.org/
   Mainnet block explorer: http://explorer.bitcoingold.org/
   Testnet block explorer: https://test-explorer.bitcoingold.org/insight/blocks

Added forum post:
https://forum.bitcoingold.org/t/network-upgrade-at-block-height-536200-approximately-july-1st-2018/1643