Ok, this is freaky.

It freaked me out, too, especially with that menacing music.

I found out how it works, though I don't think I would have ever figured it out myself.

Steven

0,9,18,27,36,45 etc. are all multiples of 9. They all have the same symbols. The answers are always multiples of 9.

They change the symbols for each trial, but they are still the same numbers.

To explain why they always end up with multiples of nine, using algebra:
Let the number for example be 52.
Let x=5
Let y=2
Then the number is 10x + y.
You are asked to add the digits.
That's x+y.
Then substract that from the original number.
That's 10x + y minus x minus y
Result is 9x which is a multiple of 9.
Try any number. The result is always a multiple of 9.

so how do they gee the correct symbol then?

In any one trial, your answer will always be a multiple of 9. All multiples of 9 always have the same symbol in any one trial. Therefore, in any trial the gopher just says what that symbol is and he'll be right. In the next trial, the gopher says what the symbol allocated to all multiples of 9 that time is, and he'll be right again, because once again your answer will always be a multiple of 9.

Unless you're lousy at math. Then the gopher gets it wrong.

So that's how I managed to fool the gopher! I knew being lousy at maths must have some advantages!

Is there any way to actually not get a multiple of 9??

Sloppy math.