gravity - Stephen Hawking says universe can create itself from nothing, but how exactly?

Stephen Hawking says in his latest book The Grand Design that,

Because there is a law such as gravity, the universe can and will create itself from nothing.

Is it not circular logic? I mean, how can gravity exist if there is no universe? And if there is no gravity, how can it be the reason for the creation of universe?

Also, if the universe doesn't exist, how can it create itself? The very sentence doesn't make sense to me. It seems so absurd and illogical that I've never heard such sentences even in philosophy. On what grounds does Stephen Hawking claim this?