[GearSwap] enable/disable command always output error message

`gs enable main` output this.
```
GearSwap: enable error, passed an unrecognized slot name (0).
```

disable() and enable() are called with a couple of values which returned from string:gsub() that are replaced string and found index.
So second argument is almost always 0, they never find the slot named "0".
Since you didn't declare `_` local, you introduce a new global variable called `_` at this point.