Perfectly possible to use GG to fund elsewhere, I have been using it to fund my Talkmobile sim only deal at £7.50 a month for 18 months or so not spent a penny with gg since then.
Now my wife has to move off TM PAYG this month and as a low user with no data use because she uses an old dumb Nokia gg is by far the loser to ASDA because of the cost of true PAYG on gg. GG is not a true PAYG as it want people to sign up to monthly renewable contracts.
I want gg to be the winner and it be the one I transfer my wife too but it simply does not compete. (BTW decided to not go to 1p mobile which would be the cheapest fior her by a mile as I don't want to have to mover again in a few months if it fails).
BTW I have 2 dual sim phones running all 4 netwroks 3, 02 (gg) Voda (TM) and EE (ASDA)