The Min MTU for a GIF interface is 1280.
#define GIF_MTU_MIN (1280) /* Minimum MTU */
There are some cases where this is still to high.
And to quote the man page:
If the outer protocol is IPv6, path MTU discovery for encapsulated
packets may affect communication over the interface. The first bigger-
than-pmtu packet may be lost. To avoid the problem, you may want to set
the interface MTU for gif to 1240 or smaller, when the outer header is
IPv6 and the inner header is IPv4.
So I would expect that it is possible to set the MTU for a GIF interface lower than 1280.
it's according to the ipv6 rfc 8200
IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.