I found that the long bristled side of a natural bristle fingernail brush (plastic bristles are usually too stiff) is the best at getting Tire Gel spread evenly over the face and into all the lettering of the tire. I add a thin bead of Tire Gel (like toothpaste) to the brush and spread as much as possible (until there is no excess in the letters and grooves).
Almost no waste because the bristles don't really absorb much, you get a nice even thin coat without the build up around the letters (which helps against sling). Plus, I swear I get better durability with this method as well (the trick to durable Tire Gel is not using a Tire Cleaner every wash. I just pressure rinse the tires and leave 'em be, looks great, it only actually needs cleaning and reapplication when the tire loses the look you want... right?) The rims get brushed with just car wash solution, a good Wheel Wax makes that a snap.
BTW, I use Opti-Bond and DP Tire Gel :dblthumb2:
Notice that the bristles are cleaner where they contact the tire. IME, applying Tire Gel to very clean and dry tires is the most important part. Spreading Tire Gel over dirty grooves and lettering won't look good for long.