The tutorial is a bit misleading. It is mixing up two packages: wiringpi and raspi-gpio. Wiring Pi has the gpio command and can be installed with sudo apt-get install wiringpi
pcmanbob has already pointed out the other one.
Note however, for the "full" Raspbian distro. wiringPi
is (should be) already installed (W.r.t. Raspbian Lite IJDK). A (quite useful) quick test is gpio readall
** FWIW, some example outputs for various Pi's are here: http://www.cpmspectrepi.uk/raspberry_pi ... adall.html
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm