In Unix and Unix-like systems (like Ubuntu), archiving and compression are separate.
tar puts multiple files into a single (tar) file.
gzip compresses one file (only).
So to get a compressed archive, you combine the two, first use
pax to get all files into a single file (archive.tar), then
gzip it (archive.tar.gz). If you only have one file you need to compress (notes.txt), there’s no need for tar, so you just do
gzip notes.txt which will result in
notes.txt.gz. There are other types of compression, such as
xz which work in the same manner as gzip (apart from using different types of compression of course)