Basically, this is a smarter version of the standard cal program from util-linux.

While cal 7-9 gives you this:

cal: failed to parse timestamp or unknown month name: 7-9

with gcal 7-9, you get:

      July 2017
 Su Mo Tu We Th Fr Sa
                    1
  2  3  4  5  6  7  8
  9 10 11 12 13 14 15
 16 17 18 19 20 21 22
 23 24 25 26 27 28 29
 30 31

     August 2017
 Su Mo Tu We Th Fr Sa
        1  2  3  4  5
  6  7  8  9 10 11 12
 13 14 15 16 17 18 19
 20 21 22 23 24 25 26
 27 28 29 30 31


    September 2017
 Su Mo Tu We Th Fr Sa
                 1  2
  3  4  5  6  7  8  9
 10 11 12 13 14 15 16
 17 18 19 20 21 22 23
 24 25 26 27 28 29 30

It also knows about many international and religions holidays, and can tell you the phases of the moon. And lots of other fancy stuff.