When I run the route command, the default gateway resolves to _gateway. I can ping _gateway, which resolves to my default-gateway and replies as such.
Why does Ubuntu 18.04 prepend _ to gateway?
Taken from systemd System and Service Manager, "CHANGES WITH 235":
nss-myhostname/systemd-resolved will now by default synthesize an A/AAAA resource record for the "_gateway" hostname, pointing to the current default IP gateway. Previously it did that for the "gateway" name, hampering adoption, as some distributions wanted to leave that host name open for local use. The old behaviour may still be requested at build time.
As you can see, some distributions objected to the use of
gateway as an automagic alias so it was changed to
systemd-resolved documentation, "Synthetic records".