Follow official instruction from this GitLab article : https://gitlab.com/help/user/project/repository/gpg_signed_commits/index.md
based on my experience, if you're linux user I think that step is enough
if you get error like this
fatal: failed to write commit object```
then you might follow optional step:
Get your `signingkey` in this way.
```brew install gnupg gnupg2 pinentry-mac
git config --global user.signingkey <YOUR_SIGNING_KEY>
git config --global commit.gpgsign true
git config --global gpg.program gpg```
Put the following in `gpg.conf` file (edit file with `nano ~/.gnupg/gpg.conf` command):
```no-tty```
Put the following in `gpg-agent.conf` file (edit file with `nano ~/.gnupg/gpg-agent.conf` command):
```pinentry-program /usr/local/bin/pinentry-mac```
You might need to execute `killall gpg-agent` command after editing the configurations file, `gpg.conf`, according to the comments. As the self-explanatory command says, this command will terminate the GPG (Gnu Privacy Guard) agent.