My own plan for a Gate defense runs something like this:

Step 1: Very publically "go mad". Pretend I've gone Halaster Blackcloak when I, in fact, retain as much of my sanity as I've ever had.

Step 1-A: Acquire spells, powers and gizmos as needed to be able to completely cloak myself from attempts to scan or locate me using magic, psionics, incarnum or whatever's going.

Step 2: Unleash a series of earthquakes and volcanic eruptions in a wide area around the location of the Gate. These must be precisely calibrated to rearrange the landscape while not impacting the Gate.

Step 2-A: While the earthquakes etc. are still ongoing, construct layers of earth and stone over the gate, effectively burying it. Before burying it, coat the thing in lead and cast permanent abjurations on it to make sure it can't be magically detected.

Step 3: Use the Origin of Species epic spell to create a species of mind-numbingly dull humanoids. They're not good, they're not evil, they're just blah. They're content to lead dull lives, doing repetitive jobs in a dull place. Have them set up villages and farms on the land over the Gate.

Step 4: Very publically construct a dungeon in one of the new mountain ranges raised during step 2, at perhaps a week's travel from the dull people's homes. Fill dungeon with vicious traps, more vicious beasts, and set up spells to summon, burn, freeze, slice and dice to taste. Appoint the cruelest creatures that will work for coin to serve as dungeon commanders, and tell them that they need to guard the innermost chamber.

Step 5: Ward innermost chamber with every abjuration spell available, then place mirrors of opposition on every available surface. Lock and bolt innermost room and set the most powerful creature you can hire or summon to guard it.

Step 6: Create a private demiplane using the Genesis epic spell, then set up shop as a scholar / healer in the dull people's town. Sit back, relax and watch creatures interested in the Gate pass through, pay the dull people for supplies, then march off to hurl themselves into the hideous meatgrinder you've set up for them. See also step 1-A.

Step 7: Perform maintenance as necessary. If some whackjob lich starts murdering the dull people before going into the dungeon, retreat into previously-prepared demiplane and wait. When he leaves, come back out and restock dull people and meatgrinder for the next chump to come along.