There wasn't - the largest minimum integer size from C90 (ANSI C) was long, with at least 32 bits. "long long" was agreed upon at an informal summit in 1992 as an extension 64 bit type on 32 bit systems until it was standardised in C99 (but already existed in several compilers at that point, including GCC).
So GCC may have had 'long long' already when Linus started working on Linux.
So GCC may have had 'long long' already when Linus started working on Linux.