"Future Trunks: My power! Be a shield to this world...to the Humans!
Father-Son Galick Gun is an attack used by Future Trunks and Vegeta. It was formed when they stood side by side and both fired a Galick Gun together. They were close enough that the two blasts combined into one giant blast.
This technique was first used to counter Fused Zamasu's Holy Wrath. At first, Future Trunks just fired off a single Galick Gun, but then Vegeta joined him and fired his Super Galick Gun as well. They were so close that the two Galick Gun blasts combined into a bigger blast and was strong enough to overwhelm Fused Zamasu's attack.