To gzip a file while keeping a copy of the original file use the following command:

gzip -c -- file1.ext > file2.ext.gz

Also, you can define a shell function with the following code:

function gzipkeep {
if [ -f "$1" ] ; then
gzip -c -- "$1" > "$1.gz"
fi
}
and then

gzipkeep file.txt