I don't think GMTV will take the ITV branding, not yet anyway. GMTV is just being treated like a programme now, at the most I think ITV News will provide the news updates from ITN rather than GMTV News.
IF that happens, the production will be based where it currently is. ITN involvement is not confirmed by any means. ITN is a third party company, not owned by ITV, and can't at present appear to deliver what ITV require at the price
well ITN is 40% owned by ITV IIRC. So I should imagine that ITV have quite a bit of a say to if and how much ITN will produce coverage for GMTV.
It doesn't really work like that...
ITV plc will have a budget for the news provision of GMTV. ITN will have a price for what ITV plc want for GMTV. If ITN can't meet ITV plc's requirements at a given price, then ITN won't get the work. Just because ITV plc own a chunk of ITN, doesn't mean that they can force ITN to take a contract at a loss.
I suspect that they will be looking at what facilities, production and personnel ITN can make available to GMTV and at what cost, and ITN may increase what they already do for GMTV (ISTR that ITN replaced Reuters as a news provider for GMTV a while back?) However this doesn't mean that ITN will definitely take-over full news provision within GMTV, though I can see a situation where more ITN reporters appear on GMTV, along with ITV News regional reporters? That kind of joined up thinking makes sense for ITV plc as a whole?
(You have to be a bit careful to differentiate between ITV plc - who own the England and Wales licences of the ITV/Channel 3 network, and now also the GMTV nationwide licence - and the ITV Network, which also includes STV, UTV and Channel, who are not part of ITV plc, but are also shareholders in ITN ISTR?)