echo to gid_map fails but uid_map success

I'm trying to map the user and group ids in new namespace by writing to uid_map and gid_map files. So on **terminal-1** I'm doing

vaibhav@vaibhav:~$ unshare -U /bin/sh
$ id
uid=65534(nobody) gid=65534(nogroup) groups=65534(nogroup)
$ echo $$
2506

then I open new terminal i.e. **terminal-2** and I do

vaibhav@vaibhav:~$ echo '0 1000 1' > /proc/2506/
uid_map
vaibhav@vaibhav:~$ echo '0 1000 1' > /proc/2506/
gid_map
-bash: echo: write error: Operation not permitted

now if I check on **terminal-1**

$ id
uid=0(root) gid=65534(nogroup) groups=65534(nogroup)

I know we can write to uid_map & gid_map file only once but it is failing on first write itself. I want to know why writing to gid_map failing. I'm using

Mint 20.3