i believe i've got a fix for the guncaster problem.
Someone64 and eharper256 both shown what the problem is.
the errors that pop up is the result of a conflict with the sprite names with the champion icons and something
in Guncaster. I couldn't actually find the conflicting sprites in Guncaster itself, though.
The fix for this is to simply change the name of the champion icon sprites to something that does not conflict with Guncaster.
I started this by going into the "sprites/icons" folder and changing the sprite names from "ICON##" to "MCON##", as you can see in the below screenshot:
Next, I went and found any reference to the icon sprites and changed them to match the new names I gave them.
After I had done that, it worked like a charm! I no longer got the errors in the console, and the game didn't crash! The only other hiccup I encountered is that if champions is loaded after Guncaster, it overrides Guncaster's bloodspot changes, but loading Guncaster after Champions fixed that. Here's a link to my copy which already has the changes.
And here's a screenshot to show that it now does indeed work with Guncaster.