If you go to http://www.thefreedictionary.com/natter then you see the definition: -
intr.v. nat·tered, nat·ter·ing, nat·ters
To talk idly; chatter.
[Variant of dialectal gnatter, to nibble, natter.]
natter [ˈnætə] Chiefly Brit
(intr) to talk idly and at length; chatter or gossip
prolonged idle chatter or gossip
[changed from gnatter to grumble, of imitative origin; compare Low German gnatteren]
v natter [ˈnӕtə]
to chatter or talk continuously, usually about unimportant things.
Basically it means to chat/talk etc, the 'G' is used at the front because when you look through the forum, quite a few people (myself included) will add a 'G' in front of an 'n' to parody the Gn15 scale definition
Hope this helps