The following does a relational JOIN on the two files on the GID and outputs the username and group name with a : in-between:
join -t : -1 4 -2 3 -o 1.1,2.1 \
<( sort -t : -k4,4 /etc/passwd ) \
<( sort -t : -k3,3 /etc/group )
The GID is in column 4 in the passwd file and in column 3 in the group file.
With -1 and -2 we specify what the join field is in the two input files and with -o we specify what fields we want to output from each file (the first field of each file).
The sorting of each file on the join field in necessary for join to work.
To get this in the exact format requested, just pipe it though sed 's/:/, /' which replaces the colons with comma and space.