Gamified course: how to deal with the massive amount of grading needed without using specific software?
I'm a former gaming engineer (many years ago), now college CS lecturer. Personally, I resist attempts at "gamification" of courses -- in short, it seems to me like a corruption of the gaming concept, and something that weakens both sides of the enterprise. 

It sounds like you're struggling to get your arms around a desired major overhaul of the course. I think that many "reform" proponents would suggest that you take smaller steps, trying out some small single component in a course, assessing how well that works, and then iterating. Also consider: Will future courses use this gamification approach? Or should you be preparing your introductory students for the expectations of "normal" college courses with which they will be interacting in the future? 

My top suggestion, as a first step, would be to work in some programming assignments which the instructors do manually grade throughout the semester. Feel free to make the assignments fairly simple and standard; directly from the book is fine. Have programs that take obvious input and output some kind of deterministic result -- write a simple batch file to compile and help test the functionality. Run them through an automated plagiarism checker (Stanford Moss). Also read the code and give feedback/points for proper style. I do this for ~8 assignments per semester, ~25 students per section. If you have double the students, then maybe halve the number of assignments. 

Personally I think that you've got to commit to some kind of personal assessment of actual code for a proper programming course. Students that don't get that and wind up in my programming 2 course very much struggle at that level. Yes, this is more work than just one or two exams alone. But if students can't write code, then you should probably fail them, and if you want to think of those grades as your academic "gamification", then maybe that will help you out.