Try this:
for file in *.gz; do
gunzip -c "$file" > "${file/.DAT*/.DAT}"
done
The approach uses gunzip's option to output the uncompressed stream to standard output (-c), so we can redirect it to another file, without a second renaming call. The renaming is done on the filename variable itself, using bash substitution (match any globbing pattern .DAT* and replace it with .DAT). The loop itself just iterates over files in the current directory with names ending with .gz.