I've used column 1 in the Index function as I've reduced the range to make the formula more efficient, the range is relative so will work the same.
INDEX('GS Auswertung - 1'!$A$2:$AC$200;MATCH($A6;'GS Auswertung - 1'!$A$2:$A$200;0)+1;COLUMN(B$1)+1))))
Column(B$1)+1 is the same as Column(C$1), by changing the Index to 'GS Auswertung - 1'C$2:C$200 you are eliminating all of the columns not needed, as you copy the formula to other columns, that range will change the same as your existing Column(B$1)+1 so you always want the first and only column in the range.
You don't even need the ,1 at the end of the Index, I just left it in to keep things tidy.
Which cell have you entered the formula in? If you're getting circualar reference from it then that means that the formula is trying to get a value from the cell that it is contained in, and is also the reason why you get a 0 value result.
The countif is there to simplify the formula, it serves the same purpose as the ISNA(....) section in your original formula (ISNA is the translation of ISTNV).
