What I currently do is open a addon's .gma file as an .txt and edit the delay between the fire, but once I do that, the whole file is erased once I go back on Garry's Mod. And when I play Garry's Mod again, the file comes back, but to it's default settings.
.GMA files are special files (you can kinda compare them to zip files).
Whenever you start gary's mod, the game unpacks these files and loads their resources.
Therefore you should not alter these .GMA files directly.
If the addon does not present you with a config file, there is not a lot you can do except to unpack the .GMA, alter the content and re-upload the file again to the steam workshop under your own name.
Though, when an addon does not provide a way to alter these settings, generally there shouldn't be a way to alter them.