The gun (more specifically the cards) target people who have an interest in the MC, mostly as they are familiar with a fictionalized version of them from their own universe. It's not brainwashing - they're simply taking people from the infinite multiverse who already feel that way and are willing to help fight.
ch 0 -2 Harem Information (Spoilers)
Anime & Comics · DWS
You approach and realise that both machines have been turned on and used. When you finally notice what Rias has actually done, she looks like a kid caught with her hand in a cookie jar. Rias used the printer while you were away. The card laying in the output chute isn't of any of the girls you have considered recently, but rather one of her brown haired, big breasted mother - Venelana Gremory.
Anime & Comics · DWS
You turn to Mitsuru and hold up the card, "What's this? Did you just drop a trading card in here for dramatic effect?"
Anime & Comics · DWS
Mitsuru does not air any objections. She simply begins the process anew. Lala pays rapt attention as she works at making the requested card a reality. Again, the press is slammed down, and again you are handed a new card. This one bears the smiling face of the serene yet dominant Akeno.
Anime & Comics · DWS
An eccentric looking girl wearing a strange white and purple outfit, with pink hair and green eyes. A devilish looking tail flails behind her. "LALA" is written in the bottom right corner just like the last card.
Anime & Comics · DWS
( ͡° ͜ʖ ͡°)
"My father had two dozen wives before he passed away," Rias explains, "Of course, only my mother was graced with children - as the lady of the house." Akeno sees fit to elaborate no further with her own… family circumstances.
Anime & Comics · DWS
That's a little out of the scope of what I've written (so far,) but Mitsuru never says no to an ambitious idea - with enough mastery of the technology it's entirely possible he could in the future.
Before she disappears back into her trading card prison, Rias bends down and kisses you on the cheek, "Don't feel so bad, you're my cute little summoner after all." Her hands cup your cheeks and run through your hair in an affectionate and sisterly way, much like she did to Issei in their anime. Rias is always eager to shower people with affection and words of reassurance after all.
Anime & Comics · DWS
They're on the shortlist!
ch 35 Changing Tides
Harem Hero
Anime & Comics · DWS