Explore the Underdark

Search your library for up to two basic land cards and/or Gate cards, put them onto the battlefield tapped, then shuffle.
You take the initiative.

Gates Ablaze

Gates Ablaze deals X damage to each creature, where X is the number of Gates you control.

Gatekeeper Gargoyle

Flying
This creature enters with a +1/+1 counter on it for each Gate you control.

District Guide

When this creature enters, you may search your library for a basic land card or Gate card, reveal it, put it into your hand, then shuffle.

Azorius Guildgate

This land enters tapped.
{T}: Add {W} or {U}.

Crackling Perimeter

Tap an untapped Gate you control: This enchantment deals 1 damage to each opponent.

Rakdos Guildgate

This land enters tapped.
{T}: Add {B} or {R}.

Boros Guildgate

This land enters tapped.
{T}: Add {R} or {W}.

Golgari Guildgate

This land enters tapped.
{T}: Add {B} or {G}.

Orzhov Guildgate

This land enters tapped.
{T}: Add {W} or {B}.

Maze's End

This land enters tapped.
{T}: Add {C}.
{3}, {T}, Return this land to its owner's hand: Search your library for a Gate card, put it onto the battlefield, then shuffle. If you control ten or more Gates with different names, you win the game.

Garrison Sergeant

This creature has double strike as long as you control a Gate.

Archway Angel

Flying
When this creature enters, you gain 2 life for each Gate you control.

Sunspire Gatekeepers

When this creature enters, if you control two or more Gates, create a 2/2 white Knight creature token with vigilance.

Armory Guard

This creature has vigilance as long as you control a Gate.

Gatecreeper Vine

Defender
When this creature enters, you may search your library for a basic land card or a Gate card, reveal it, put it into your hand, then shuffle.

Gruul Guildgate

This land enters tapped.
{T}: Add {R} or {G}.

Gatebreaker Ram

This creature gets +1/+1 for each Gate you control.
As long as you control two or more Gates, this creature has vigilance and trample.

Simic Guildgate

This land enters tapped.
{T}: Add {G} or {U}.

Ogre Jailbreaker

Defender
This creature can attack as though it didn't have defender as long as you control a Gate.

Gate Colossus

Affinity for Gates (This spell costs {1} less to cast for each Gate you control.)
This creature can't be blocked by creatures with power 2 or less.
Whenever a Gate you control enters, you may put this card from your graveyard on top of your library.

Tireless Tracker

Landfall — Whenever a land you control enters, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")
Whenever you sacrifice a Clue, put a +1/+1 counter on this creature.

Dimir Guildgate

This land enters tapped.
{T}: Add {U} or {B}.

Izzet Guildgate

This land enters tapped.
{T}: Add {U} or {R}.

Wojek Investigator

Flying, vigilance
At the beginning of your upkeep, investigate once for each opponent who has more cards in hand than you. (To investigate, create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")

Nine-Fingers Keene

Menace
Ward—Pay 9 life.
Whenever Nine-Fingers Keene deals combat damage to a player, look at the top nine cards of your library. You may put a Gate card from among them onto the battlefield. Then if you control nine or more Gates, put the rest into your hand. Otherwise, put the rest on the bottom of your library in a random order.

Basilisk Gate

{T}: Add {C}.
{2}, {T}: Target creature gets +X/+X until end of turn, where X is the number of Gates you control. Activate only as a sorcery.

Guild Summit

When this enchantment enters, you may tap any number of untapped Gates you control. Draw a card for each Gate tapped this way.
Whenever a Gate you control enters, draw a card.

Open the Gates

Search your library for a basic land card or Gate card, reveal it, put it into your hand, then shuffle.

Selesnya Guildgate

This land enters tapped.
{T}: Add {G} or {W}.

Ubul Sar Gatekeepers

When this creature enters, if you control two or more Gates, target creature an opponent controls gets -2/-2 until end of turn.

Circuitous Route

Search your library for up to two basic land cards and/or Gate cards, put them onto the battlefield tapped, then shuffle.

Baldur's Gate

{T}: Add {C}.
{2}, {T}: Add X mana of any one color, where X is the number of other Gates you control.

Saruli Gatekeepers

When this creature enters, if you control two or more Gates, you gain 7 life.

Opal Lake Gatekeepers

When this creature enters, if you control two or more Gates, you may draw a card.