Warren has released a tax calculator, according to which Bill Gates would pay $6.319 billion on his $106 net worth in wealth tax (which approximately matches her proposed wealth tax of 6% for >$1b annually).
Various sources put the net worth of Gates at $110 billion.
In either case, Gates would have ~$100 billion left after the wealth tax. For that matter, he would have ~$10 billion left if the $100 billion number were true.
This does not consider existing taxes or other tax increases in addition to the wealth tax (such as an additional 3% for a medicare for all plan, or handling capital gains tax the same as income tax; but there's no reasonable way to reach the claimed $100b number; which is probably why Gates qualified the claim with "I'm just kidding")