The dictionary definitions of the two might be similar, but there is a big difference in connotation and modern usage.

I would say the focus is on opposite sides of the spectrum. When you are defining a group, the primary focus is on who should be included. The focus of gatekeeping is on the people who should be excluded. This is usually done in order to raise the status of those who are included or those who are setting the rules for inclusion. Those rules are often arbitrary or draconian and are usually set by some central authority (or someone who feels they have such authority). This authority will then be the one who doles out permission for others to participate.

That is where the software developer example comes in. Anyone can join the open source software development community and they will only be judged on the merit of their contribution. They might have never contributed to any OSS. They might have never been paid to develop software. They might have no formal education in software development. None of that would matter if we are defining people based on the merit of their work. However they would likely be excluded in a system that relies on gatekeeping.

A definition of journalists that excludes small scale bloggers, and such would create a different population from say, one that comprised "anyone who self identifies as a journalist", for instance.