In iCal can't assign .ICS calendar event to non-"on my mac" calendars

In apple mail, I click on the the .ICS file [sent by windows outlook user] and can see that it is added into iCal. Great, as expected.

However, if I right click on this event, under "calendars" I can only assign it to either "home" or "work" i.e. those same calendars that show up under the "on my mac" section, top left of iCal - no other calendars show up

When I select different events already in the calendar (and not added using .ICS files), I can see more calendars, namely those added from gmail

e.g. bob.smith@gmail.com

= vacation

= work

= training

jan.smith@gmail.com

= bookings

= vacation

these are the same ones which appear under the 'on my mac' section

How can I assign this .ICS event to one of my gmail calendars?

anyone know how to fix this?