It's not a paid add-on... GnuPG is open source. They are selling a support plan in case you need help with open source.
Here is the same thing, without the paid support. https://github.com/Free-GPGMail/Free-GPGMail
- GPG Suite was released under an Open Source license in the past. Is that stil the case?
You GPG Suite including GPG Mail is still going to be released under an Open Source license.
- Am I still allowed to compile my own version of GPG Mail / GPG Suite removing any code regarding the trial or activation?
You absolutely are, the GPL makes sure of that. You will find the source code on this website next to the download link for GPG Suite.
We would kindly like to ask you not to use our names or icons if you plan to publish a binary for others to use. Also please do not release any complete installers as that may suggest that they are official releases.
https://gpgtools.org/faq