Unfortunately the Target for this alert is Host - not Datastore. So you cannot use a group of Datastores for the override.
However, fortunately I anticipated this type of request
The rule vSphere Host Storage Error
has overrides that allow specific event IDs or text to be excluded. If you examine the available overrides, there is one called 'Exclude Text'. This is already populated with contents -DummyEventText1|DummyEventText2
(this dummy text is included just to show the format of the text, and that multiple strings are supported using RegExp)
This parameter supports wildcards, so in this case replace the dummy text with the desired wildcard -*_GX_BACKUP_*
This will prevent the rule firing when event description contains _GX_BACKUP_
That should do it! Let us know how it goes,