Well, first you need that command in an event. For example, if it's an on TEXT event, you'd use:
on 7:text:*:#: { }
Just change it around as needed. The main thing is the 7 or whatever level you use (named or numerical). Personally, I like named levels for things like this because it helps to keep track of what the levels are for, but it's up to you. Both will work equally well.
A question about your code, though...
&& ($1 == %gathertegn $+ on)
Is there a reason you're adding "on" to the end of whatever the variable is? Just making sure because it doesn't make sense to me.
And, your "on CanUse" part isn't valid. You need to use a real event (on TEXT, on INPUT, on JOIN, on QUIT, etc.).
