What information do you want to extract (only stops, only schedules, stops per line, schedule per line, etc.) and how do you need it delivered - as a one time conversion or as a service? Also, are you familiar with a particular programming language?
This is a complex matter that will depend on your desired implementation strategy, agency size and available computational resources.
Also, you have asked about „GTFS to JSON” but this is the GTFS-Realtime group. These are two linked but distinct standards. What it is exactly you want to convert - „static” GTFS timetable/route description data that's given as a ZIP file/folder of multiple txt files or a real-time trip updates/service alerts/vehicle positions feed that you're grabbing as a protocol buffer from a link?
We'd be glad to assist you here on the group but much more context is needed to offer helpful advice.
Have a nice evening!