Help, please! I am summing alternate rows in a spreadsheet, e.g.

=SUMPRODUCT(--(MOD(ROW(F5:F306),2)=1),F5:F306)

but I only want to include the rows where is a particular bit of text in Column C (in this case "GARD"):

=SUMIF($C7:$C302,"GARD",G7:G302)

Is there a formula which will add these selected rows only?

Many thanks.