gawk '/>/ {$0=substr($0,2); sid=$1; next} \
          {sl[sid]+=length($0); gccount[sid]+=gsub(/[GgCc]/,"X",$0)} \
      END {for(i in gccount) printf "%s\t%.2f\n",i,gccount[i]/sl[i]}' \
      mysequences.fasta > gc_count.csv