How do I calculate how much gross margin for each moth?

Sales : Jan-12000; Feb-8,000; Mar-13,000; Apr-11,000; May-9,000; June-13,500

Collection: Jan-11,000; Feb-10,000; Mar-11,500; Apr-10,500; May-10,500 June-9,500

Cost of Goods sold is always 65 percent of sales.

Thank u.

GM is revenues minus COGS, or when done as a percentage: (Rev - COGS)/Rev.