See "Gladiator Beast Alexander" for additional rulings.
If “Gladiator Beast Laquari” is Special Summoned by the effect of a “Gladiator Beast” monster, its original ATK becomes 2100. If “Skill Drain” is later activated, this effect is negated and “Gladiator Beast Laquari’s” original ATK will return to 1800. Even if “Skill Drain” is later negated or removed from the field, the original ATK of “Gladiator Beast Laquari” will remain at 1800.
If “Gladiator Beast Laquari” is Special Summoned by the effect of a “Gladiator Beast” monster and it is flipped face-down, when it is flipped face-up its effect will not be reapplied so its original ATK will be 1800.