(thanks for the tutorial on posting code btw, will edit my previous posts)
Yes the "-add "$k part is G’MIC specific (the k={6*($<+1)} creates a variable that is used here) and if I echo the lines with backslashes and with single quotes the output is the same (so nothing get eaten or changed by bash as far as I can tell) but G’MIC refuses the line with single quotes.
Funny enough the earlier part ({$k} after rotate command) does work with single quotes