The effect of this card only activates when you elect to change its Normal Summon to a Special Summon. Not if it is Special Summoned by “Monster Reborn”, “Marauding Captain”, etc.
You can use the effect of this card to change its Normal Summon to a Special Summon even if your opponent has no monsters in their Graveyard.
Netrep Rulings:
“Disappear” can be chained to the effect of “Gilasaurus” to remove the monster chosen to be Special Summoned from the Graveyard from play.
If “Necrovalley” is active on the field, “Gilasaurus” may still be used as a Special Summon. The opponent may not Special Summon a monster from the Graveyard from “Gilasaurus’” effect as a result of the effect of “Necrovalley.”
If “Skill Drain” is active on the field, “Gilasaurus” may still be used as a Special Summon. The opponent may not Special Summon a monster from the Graveyard from “Gilasaurus’” effect as “Gilasaurus” effect is negated by “Skill Drain.”