The Bitcoin Gold team made an announcement today clarifying some points about their 'pre-mined' endowment of 100,000 BTG.
In their statement they write that the pre-mine was actually a post-mine taking place after the fork, during which they mined 8,000 blocks each earning 12.5 BTG coins. 95% of these coins are locked up in an endowment for long term holding according to the team. The remaining 5% is distributed for core development team of 6 people, 833 BTG coins each.
See the official statement here: https://bitcoingold.org/premine-endowment/
Bitcoin Gold chose to fork (replicate) the entire existing Bitcoin blockchain, with all wallet and transaction history included, and then later begin a brief project mining period to build a store of coins to support the future of the project. This happened after bitcoin block 491,407 (2017-10-24 01:20:39). At that point, there were already 16.5 million Bitcoin in existence, so there were then 16.5 million Bitcoin Gold in existence.
After this, the project mined 100,000 coins (8000 blocks at 12.5 coins per bock), so it’s more of a post-mine, but we still call it a premine. A true pre–mine is when the makers of coin start a new blockchain with a genesis block and do private mining to accumulate a number of coins, and only then make mining public for others earn coin rewards. With that approach, the makers obviously start out with a huge proportion of the initial coin supply – 100%! That’s would have been a serious problem.
See the official statement here: https://bitcoingold.org/premine-endowment/