GMEdit makes backups automatically whenever hit Ctrl+S - you can access them by right-clicking any tab.
I think the primary thing that results in GMS1.4 overwriting changed files is opening them in GMS and then closing while saving changes - this stores the file in the memory and it usually will no longer check for changes on disk and may decide to overwrite the current version with the one from memory when saving. As per other topic, this happens more rarely if you have version control enabled for the project (even if you don't actually use it)