The two creatures you tap to activate the ability don’t necessarily have to share a creature type, but the creature card you put onto the battlefield must share a creature type with each of them. For example, you could tap an Elf Wizard and a Human Warrior to put an Elf Warrior card onto the battlefield.

You can tap any untapped creatures you control to activate Cryptic Gateway’s ability, even ones you haven’t controlled continuously since the beginning of your most recent turn.