I am also a bit rubbish with Excel :-)

But, perhaps something like this would work:-

=IF(D2=1;A2+A5;0) returns -6
=IF(D5=1;B2+B5;0) returns 4
And
IF(D6=1;B3+B6;0) leaves the cell blank

So when they are strung together, the Gross Data B2:B8 column would look like this:-
=IF(D2=1;B2;0)+IF(D3=1;B3;0)+IF(D4=1;B4;0)+IF(D5=1 ;B5;0)+IF(D6=1;B6;0)+IF(D7=1;B7;0)+IF(D8=1;B8)

Perhaps someone with some skills can suggest a more "elegant" solution :-)