I started out on gel too, changed to shots, and am now back on the gel due to acne problems.
Showering right before applying the gel ensures your skin is most accessible for the gel. You open up the pores and whatnot with a hot shower, and you remove what residue was left from the previous application. Showering before application is not a must, but it surely creates more perfect circumstances for the gel to be absorbed.
Sure, most of the gel is absorbed rather quickly into the skin after application, and is then slowly released into your system. But it is advised not to shower for 6 hours after application because you will rinse off what was left on the surface of the skin. So to be safe, don't shower for at least a couple of hours after application.
Hope this helps and congrats on starting T!