Fretta game mode problem?

I am pretty new at creating dedicated servers and I am having an issue with voting at the end of a match. I am trying to create a prop hunt server but when the players vote at the end of a match if you click on prop hunt it just shows a small overlay that says nil. I tried looking in the directory for sv_gmchanger.lua but I couldn’t find it. Here is a screenshot of the error.

http://s24.postimg.org/mspq8a6zp/problem.png

http://s23.postimg.org/i6i2w7wzf/problem.png

Thanks!

I didn’t realize fretta wasn’t a part of gmod. I downloaded a workshop that had the voting system and I included it in my collection and it worked.