Vegeta's Galactic Gunfire starts out with a launching kick (which you can combo into, recommended to cancel it off a Rising Knee Arrow).
When it hits, the opponent is launched into the air. Vegeta teleports above them, stomps them back down and then starts charging up energy.
During this part, two icons will appear onscreen beside Vegeta indicating that the player can mash buttons to speed up the energy charging;
doing so will result in more Ki Blasts being fired down.
Without the button mashing, the move isn't very damaging, so mash like you mean it!