By Lili Bayer and Andrew Gray BRUSSELS, July 6 (Reuters) - NATO leaders will gather in Ankara for a summit on Tuesday and Wednesday, amid pressure from President Donald Trump for Europe to step up defence spending and following months of transatlantic friction over the Iran warand Greenland. The U.S.. president's frequent criticism of NATO, along with announced troop withdrawals from Europe and a six-month review of the U.S.