Just got it done. Here is the part that I've used in the query for month-wise average salary:

AVG(CASE WHEN (m.GrossSal + m.SkillBns) > 0 AND DATEDIFF(MONTH, m.Joindate, GETDATE()) >= 0 AND DATEDIFF(MONTH, m.Joindate, GETDATE()) <= 6 THEN ISNULL((m.GrossSal + m.SkillBns), 0) END) AS [0 - 6 Months AvgSal]

Thanks all for the replies. Have a great day!