A gul rune is a mid tier rune that can be socketed into armor and weapons. You can cube 2 ist runes and a sapphire together to get a gul rune. You can get a gul rune from the hell hellforge quest.
A gul rune requires level 53 to use.
Horadric cube recipes with gul runes
transmute 2 gul runes and a ruby to create a vex rune