SUMPRODUCT works by multiply numbers (row-by-row) and then adding those
individual products up. Your numbers can be actual values and/or logical
expressions. The reason your formula works is because the logical expression

   (Main!$G$4:$G$2885 = "I")

evaluates to either TRUE or FALSE which, when used in a mathematical
expression (your multiplication for example), is converted to 1 or 0 in
order to be used in that calculation. So, if the value the logical
expression is multiplied by is TRUE, the rest of the values in the
mathematical expression get multiplied by 1, which keeps them in the final
summation that SUMPRODUCT performs; and, if FALSE, the rest of the values in
the mathematical expression get multiplied by 0, the product of which is 0,
which effectively removes them from the final summation that SUMPRODUCT
performs. In effect, those logical expressions act as IF-THEN filters. So,
if you want another IF-THEN type test, just multiply its logical expression
with the rest of the items being multiplied. Hence, you want this...

=SUMPRODUCT((Main!$B$4:$B$2885 = "GL")*(Main!$G$4:$G$2885 =
"I")*(Main!T4:T2885))

Rick