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"ip tun add" fails with wrong error if remote is not specified:

root@rhel8:~# ip tun add grezz0 mode gre
add tunnel "gre0" failed: File exists

if the remote is specified, the tunnel creation works:

root@rhel8:~# ip tun add grezz0 mode gre remote 1.2.3.4
root@rhel8:~# ip link show dev grezz0                                                                                                                                                                                                        
6: grezz0@NONE: <POINTOPOINT,NOARP> mtu 1476 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/gre 0.0.0.0 peer 1.2.3.4

The error message is a bit misleading, but matches the error code propagated from the kernel. Unfortunately the kernel code path uses ioctls and is not suitable for nextlink extack error reporting.
Closing as WONTFIX for now, should this change on kernel side we can re-evaluate this for a proper extack message.