who along with gates signed a pledge to give away his entire fortune to charity when he dies

Which charities? The charities owned by them and their family of course. This is just tax dodging dressed up by good PR.

Billionaire run charities are just investment portfolios with good PR, they're only legally required to actually donate 5% of their holdings a year (and there are loopholes such as donating assets that aren't easily valued, which mean they often don't even have to donate that much).