You can check if your developer uploaded a repository on Google Cloud Source Repositories by going to your Dashboard/Console > Menu > Tools > Source Repositories or Source code. If not, here are other possible locations for your file:
1- Your developer's local machine
2- GIT / SVN repositories like GitHub, Bitbucket, GitLab, Google Cloud Repositories
Even if you were able to find the files, you would still need to know which branch and which commit was deployed from these repos. Your best option is to ask the person / company who created your site.