LONDON/SINGAPORE, March 16 (Reuters) - ** Gail India proposes swapping seven LNG cargoes across May-October 2018, according to the tender document and traders. ** The Indian importer has 20-year deals to buy 5.8 million tonnes per annum of U.S. LNG in total, split between Dominion Energy's Cove Point plant and Cheniere Energy's Sabine Pass. ** With few tankers to ferry the fuel to India, Gail has already struck swap deals for a chunk of its Sabine Pass volumes, and is extending that programme to