Is GraphQL nexus the intended replacement for GQLGen as it is code-first rather than sdl-first?

You'll want to take a look at their blog here: https://www.prisma.io/blog/the-problems-of-schema-first-graphql-development-x1mn4cb0tyl3 GQLGen is for SDL-first development. Given the popularity of Apollo Server, SDL isn't going away anytime soon.

Nexus actually generates the SDL for you instead of constructing it by hand and works great with Apollo Server!