zero pad numbers in `cal` (or `gcal`) output?
sed to the rescue:

pse@Mithos:~$ cal 09 2015 | sed -e 's/ \([[:digit:]]\) /0\1 /g' \
                                -e 's/ \([[:digit:]]\)$/0\1/g'
   September 2015
Su Mo Tu We Th Fr Sa
      01 02 03 04 05
06 07 08 09 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30


Combining the two sed commands into one is left as an exercise to the reader.