There are 2 general workflows to publish a GP Service (Web Tool) into production.
The most common would be to use ArcGIS Pro to connect to your Portal (or stand alone GIS Server) and publish. Based on your comments, it sounds like your Portal/Server is in another environment and ArcGIS Pro cannot connect directly to it, or you're perhaps developing a service ...
What I often do in these sort of cases is create a class based on a (representative) example of the JSON response (i.e. one that has most or all properties set to a value, rather than null):
Copy the JSON to the clipboard
In Visual Studio, add a new class to your project
Select Edit → Paste special → Paste JSON as classes
You may need to hand-edit some ...
You can load ESRI JSON in QGIS, from there just resave in any format you want.
If you have the local file you can just add it as vector through the Data Source Manager,
QGIS will notice it's ESRI JSON:
You can also use load it straight from the server with using a query url in the Protocol: HTTP(S). cloud, etc. Source type:
Saving to GeoJSON from here is ...