Drinks giant Diageo (LSE: DGE.L - news) is pouring fresh investment into its gin brands, Gordon???s and Tanqueray, to take advantage of a surprise surge in sales of the spirit in hard-hit economies such as Spain and the UK.The booming popularity of retro gin-based cocktails such as the French 75 in cities including Madrid and London is driving a resurgence of the spirit, predominantly made with ju