Installation on a clean system works as expected and the tool works fine as well:

# echo '{"a":"b", "b":[1,2,3]}' | gron                                                                                                                                                                                   
json = {};                                                                                                                                                                                                                                    
json.a = "b";                                                                                                                                                                                                                                 
json.b = [];                                                                                                                                                                                                                                  
json.b[0] = 1;                                                                                                                                                                                                                                
json.b[1] = 2;                                                                                                                                                                                                                                
json.b[2] = 3;

works as expected