do not use an if-then on a
sum.
You cannot conditionally summarize like this. That can be accomplished via Running Totals or shared variables but that is a different approach.
Try to reverse your thinking. What ew are doing is converting your row level data to be either the row value or a 0. Now if you sum ALL of these it would be the same total as if you had only summed some of them because you are zeroing out all of the ones you did not want to include.
Lets call the first formula "MTD_only"
if {Gf.Gf_Date} in monthtodate then {GfFnds_1.GfFnds_1_Amount) else 0
Place this on your detail section. You will see it 'zero out' all rows that were not part of this month.
now sum this formula field
this will show you the total for the currnt month only
now if you want to see it as a % add that to the summary
Does that help?