I spent an entire day searching the same topic.
Here's what I found and tested on Debian 9 + Samba AD DC 4.5.12
In order to set sharing folder permission, a somewhat local unix group or user must be present.
Option 1: Create Group with gid
I failed to map the gid after creation by net groupmap, but was able to map them at the time of creation by
samba-tool group add "Groupname with Space" --nis-domain=mydomain --gid-number=corresponding gid we just saw in /etc/group
chgrp localgroupnamewithoutspace /path/of/share
chmod 0770 /path/of/share
Option 2: Look up gid of AD group
Unix group doesn't allow space, so set group of directory to gid will work.
However, when doing ls -al it will show gid number not AD group or Unix group name. This could potentially be a problem if multiple groups are nested and assigned in the same large sharing parent directory.
wbinfo --name-to-sid "AD Group name with space"
wbinfo --sid-to-gid "copy from the ouput above"
it should look like this and you only need this part S-1-5-21-53980404-2501955692-3283166571-512
then use the gid number when setting permission by chgrp, chmod.
Noted, The above only takes care of AD group and Unix group mapping, the actual ACL and group member permission still requires to have proper SeDiskOperatorPrivilege setup, and then configured through Windows Client with account has SeDiskOperatorPrivilege in File Explorer and ADUC.