For simplicity
SSG - Static Site Generator
SSR - Server Side Rendering
Instead of GatsbyJS, I've tried NextJS while building my startup MFY. I would say GatsbyJS is a subset of NextJS. GatsbyJS is an SSG while NextJS can do both SSR and SSG.
If you're developing a site in which the content doesn't change frequently or you want the content to change on reach build, go for GatsbyJS.
But if your content is inside some API or database and on reach render it should show new data, go for SSR.
For example, consider the case of @hasnode. Every time a user visits they need to show the latest posts from DB. In such cases, Gatsby can't help. NextJS can render it from the server itself fetching from DB.
However consider a Blog or a static site, their inner contents won't change like this. In such cases, they can call some API (maybe WordPress rest API) and then create HTML files for each blog post, which will be super fast than SSR.
However, I left SSR (NextJS) in MFY, because I can easily deploy it to Netlify and at that time there were many bugs (looks like most of them are fixed now). If its SSR I've to run my own server