How to create JSON files for finite element meshes?

Packages like femgl (GitHub - mikolalysenko/femgl: WebGL finite element viewer) and GridApp (github.com/gridap/Gridap.jl) employ finite element mesh files in JSON format. How are these files created (preferably from meshes generated using GMSH (gmsh.info)). I was not able to find an appropriate conversion tool.

Thanks! Domenico