Is “-1>>5;” unspecified behavior in C?

Both are correct. Implementation defined behavior is a particular type of unspecified behavior.

Citing section 3.4.1 of the C standard which defines “implementation-defined behavior”:

1 implementation-defined behavior

unspecified behavior where each implementation documents how the choice is made

2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit
when a signed integer is shifted right.

From section 3.4.4 defining “unspecified behavior”:

1 unspecified behavior

use of an unspecified value, or other behavior where this
International Standard provides two or more possibilities and imposes
no further requirements on which is chosen in any instance

2 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

As for GCC, you’ll always get the same answer because the operation is implementation defined. It implements right shift of negative numbers via sign extension

From the GCC documentation:

The results of some bitwise operations on signed integers (C90 6.3, C99 and C11 6.5).

Bitwise operators act on the representation of the value including
both the sign and value bits, where the sign bit is considered
immediately above the highest-value value bit. Signed >> acts on
negative numbers by sign extension.

As an extension to the C language, GCC does not use the latitude given
in C99 and C11 only to treat certain aspects of signed << as
undefined. However, -fsanitize=shift (and -fsanitize=undefined) will
diagnose such cases. They are also diagnosed where constant
expressions are required.