i solved the trouble myself

here it is. maybe someone will need it
it checks gbk files in source dir, and deletes olders by modifying time if there are more than 7 gbk files
Code:
source_dir=/path/to/gbk/files
file_q=$(ls $source_dir|grep -ci ".gbk$")
if [ $file_q -gt 7 ] ; then
let fark=$file_q-7
while [ ! $fark = 0 ] ; do
sil=$(ls -rt1 $source_dir|grep -i ".gbk$" -m 1)
rm -f $source_dir/$sil
let fark=fark-1
done