Recently, I tried to encrypt my files with
my private key. I found a article on internet
which suggests --sign.

gpg --sign file.txt

After more searching and reading, I found
that gpg defaultly compresses a file. Although
a singned file contains meaningless characters,
but its compressed, not encrypted. I did
a test:

gpg -s -z 0 file.txt

"-z 0" means no-compression. A signed file
file.txt.gpg was produced.

cat file.txt.gpg

We can see the plain text.