Katniss is the face of the revolution. Gale is just some hunky-looking guy.
The propos featuring Katniss are all about showing her off to the audience. She's the one who can inspire the population; they don't know Gale, and his presence there might confuse them or make them wonder unnecessarily. A propaganda message is most effective when it's kept simple. You want to advertise Katniss to fire up the rebellion? Fine, advertise Katniss. Don't let some other guy take up half the screen, even if some of the viewers might remember who he is. You don't want people thinking, "who's that guy? oh, right, he's her brother or cousin or something from District 12"; you want them thinking, "Mockingjay! YEAH! Go girl on fire!" and getting inspired.
In fact, you could as easily ask the same question about out-of-universe advertising. Why do the posters for the Hunger Games films tend to show Katniss alone in the middle? Because she's the main character, that's why. Why aren't there any film posters with a picture of Katniss next to Mitchell? Because who the hell is Mitchell and why should he be advertised next to the main character, that's why. (I chose Mitchell instead of Gale because Gale is important in the story - probably as many random viewers in Panem remember Gale as random viewers in the real world remember Mitchell.)
