When a heavy have GRU he will be marked for death when he shouldn't. And when a heavy uses KGB replaced by the GRU it will swing slower.

Attribute says like this on GRU:
When weapon is active:
+50% faster move speed
-50% damage penalty
You are marked for death while active, and for short period after switching weapons

And when KGB is replaced by GRU the attributes says like this:
When weapon is active:
On Kill: 5 seconds of 100% critical chance
+50% faster move speed on wearer
-50% damage penalty
20% slower firing speed
-7 health drained per second on wearer

The recent big update broke a plugin called TF2items which controls the attributes on items, and as a result it causes odd attributes across most items. The only thing we can do is wait for TF2items to be updated by the author.