Terrorking Archfiend
Effect Monster (Fiend / DARK / 4 Stars / ATK 2000 / DEF 1500)
You cannot Normal Summon or Flip Summon this card unless you have an Archfiend Monster Card on your side of the field. The controller of this card pays 800 Life Points during each of his/her Standby Phases (this is not optional). When this card is specifically designated as a target of the effect of a card controlled by your opponent, when resolving the effect, roll a six-sided die. If the result is 2 or 5, negate the effect and destroy the opponent's card. Also negate the effect of an Effect Monster that is destroyed by this monster in battle.
Ok... as you can see there is a condition to "Normal Summoning" the card... but it still CAN ultimately be Normal Summoned. So as stated, you can use "Monster Gate" with it.
Examples of monsters that cannot be Special Summoned through the use of "Monster Gate":
Rare Metal Dragon (Cannot be Normal Summoned or Set)
Any Toon card, except "Toon Gemini Elf" ("Toon Gemini Elf" is the only Toon that is Normal Summoned. The rest are only Special Summoned).
Monsters with Special Summoning Restrictions (Like Spirit Monsters)
This list isn't exhaustive... but it just gives you an indication as to what will and won't work.