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.

Thran Portal

This land enters tapped unless you control two or fewer other lands.
As this land enters, choose a basic land type.
This land is the chosen type in addition to its other types.
Mana abilities of this land cost an additional 1 life to activate.

Gatekeeper Gargoyle

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

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.

Gateway Sneak

Whenever a Gate you control enters, this creature can't be blocked this turn.
Whenever this creature deals combat damage to a player, draw a card.

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.

Sage of the Maze

{T}: Add two mana in any combination of colors.
{T}: Until end of turn, target land you control becomes an X/X Citizen creature with haste in addition to its other types, where X is twice the number of Gates you control. Activate only as a sorcery.
Tap an untapped Gate you control: Untap this creature.

Saruli Gatekeepers

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

Glaive of the Guildpact

Equipped creature gets +1/+0 for each Gate you control and has vigilance and menace. (A creature with menace can't be blocked except by two or more creatures.)
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)

Heap Gate

{T}: Add {C}.
{1}, {T}: Add one mana of any color.
{1}, {T}, Tap an untapped Gate you control: Create a Treasure token. (It's an artifact with "{T}, Sacrifice this token: Add one mana of any color.")

Ogre Jailbreaker

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

Talon Gates of Madara

When this land enters, up to one target creature phases out.
{T}: Add {C}.
{1}, {T}: Add one mana of any color.
{4}: Put this card from your hand onto the battlefield.

Planar Nexus

This land is every nonbasic land type. (Nonbasic land types include Cave, Desert, Gate, Lair, Locus, Mine, Power-Plant, Sphere, Tower, and Urza's.)
{T}: Add {C}.
{1}, {T}: Add one mana of any color.

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.

Plaza of Harmony

When this land enters, if you control two or more Gates, you gain 3 life.
{T}: Add {C}.
{T}: Add one mana of any type that a Gate you control could produce.

Gond Gate

Gates you control enter untapped.
{T}: Add {C}.
{T}: Add one mana of any color that a Gate you control could produce.

Golgari Guildgate

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

Gateway Plaza

This land enters tapped.
When this land enters, sacrifice it unless you pay {1}.
{T}: Add one mana of any color.

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.

Open the Gates

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

Opal Lake Gatekeepers

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

Gateway Shade

{B}: This creature gets +1/+1 until end of turn.
Tap an untapped Gate you control: This creature gets +2/+2 until end of turn.

Dimir Guildgate

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

Way of the Thief

Enchant creature
Enchanted creature gets +2/+2.
Enchanted creature can't be blocked as long as you control a Gate.

Simic Guildgate

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

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.

Greenside Watcher

Navigation Orb

{2}, {T}, Sacrifice this artifact: Search your library for up to two basic land cards and/or Gate cards, reveal those cards, put one onto the battlefield tapped and the other into your hand, then shuffle.

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.

Sea Gate

This land enters tapped.
As this land enters, choose a color other than blue.
{T}: Add {U} or one mana of the chosen color.

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.

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.

The Black Gate

As The Black Gate enters, you may pay 3 life. If you don't, it enters tapped.
{T}: Add {B}.
{1}{B}, {T}: Choose a player with the most life or tied for most life. Target creature can't be blocked by creatures that player controls this turn.

Black Dragon Gate

This land enters tapped.
As this land enters, choose a color other than black.
{T}: Add {B} or one mana of the chosen color.

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.

Manor Gate

This land enters tapped.
As this land enters, choose a color other than green.
{T}: Add {G} or one mana of the chosen color.