Skill Drain negates the effects of all face-up Effect Monsters on the field. This means that Gladiator Beasts cannot activate their effects, such as "Gladiator Beast Gyzarus" search effect or "Gladiator Beast Bestiari" effect to equip itself with another "Gladiator Beast" monster from the Graveyard.
Without their effects, Gladiator Beasts are much less effective. They cannot search for their cards or equip themselves with other Gladiator Beasts, making them much easier to defeat.