for file in ./dir*/dir*/*GABA*.dat; do
 tag=${file%/*}
 tag=${tag} #here, I believe the value of the variable will be "dir*/dir*",
   so this is where I was thinking to remove the '/'
 tag=$(echo "$tag" | tr '[:upper:]' '[:lower:]')
mv -- "$file" "./GABA/${tag}.dat"
done