Sure, you can use DAX date/time calculations to determine this. Set the target value of the gauge to this calculated column to reflect the calculated goal.
Ex:
DAY(now()) / DAY(EOMONTH(NOW(),0)) * Goals[MonthlyGoal]
Today is June 12th.
DAY(now()) returns 12
DAY(EOMONTH(now(),0) returns 30 (number of days in June)
So the calculation would be:
current day / days in month * montly goal
Thanks,
Sam Lester (MSFT)