more like an example of over there vs. over here - it's in OED, shorter Oxford, Cambridge; first attested by OED in Partridge ('48) and Granville ('62) slang dict's. here're the "real" citations:

1970 A. REID Confessions of Hitch-hiker vi. 45 We went to a posh café...The prices were ginormous. 1976 Scotsman 20 Nov. 10/2 How about froggies filled with pot-pourri from small to gi-normous, as Just Us describe them. 1977 Economist 8 Oct. 98/3 The state company Egam, declared bust last spring,..is going to cost considerably more than the £500 billion..earmarked by the government last June, probably a ginormous £1,700 billion. 1986 Sunday Express (Colour Suppl.) 23 Mar. 70/3 Since Brands Hatch, doors have opened and it's possible to make gi-normous money.

it's just a portmanteau word, of course: gigantic + enormous. (listed as such in Wikipedia's list of ~ words.
http://en.wikipedia.org/wiki/List_of_portmanteaus)