For block rules there is never any need/point in specifying a gateway. The traffic is being blocked, so there are zero bytes to be sent anywhere - it really does not matter which gateway the zero bytes are sent to :D
That Policy Route Negation note is for when you want to pass some traffic locally but you are also using policy routing for Failover/Load-balnce to the general internet. You might have a policy-routing rule like:
IPv4 protocol any, Source LANnet, Destination any, Gateway load-balance-group
A rule like that will push all traffic arriving on LANnet out the load-balnace-group which goes out some WAN(s) to the big bad internet. Even "local" traffic will get pushed out. There is nothing in policy-routing rules to look and see "hey, the destination is a local subnet on this box, I will ignore the specified gateway".
So before a general policy-routing rule like that, you need to put ordinary pass rules for the local (and intranet) traffic that you want to pass locally.