The GNU Project was already an established thing when Torvalds started Linux. I think it would be fair to argue the GNU bootstrapped Linux even if you don't buy into the whole "you must call it GNU/Linux" argument.

And the whole GNU project was bootstrapped by what was going on at the MIT Media labs etc. etc.

the first Linux was written on Minix and used the Minix filesystem and GCC that was available on minix. So built on an established software stack and kernel.