I’ve written a short blog post describing how to add split testing (or A/B or MV testing) to Gatsby sites. Netlify can support this for sites built with any static site generator, but this example shows how to do this with Gatsby.
There is also a video walking through the entire process — from no site to an active split test shaping traffic on a production URL between 2 feature branches, including adding a custom dimension to Google Analytics to measure the different variants. It takes less than 10 minutes from start to finish.