What are the last 2 gates for on Animus Island

From this crafting guide:

Bomb Effects

Caltrop – Caltrop Bomb (Tactical) – Slows and stuns enemies by covering an area of ground with metal spikes. Perfect for stopping pursuits and blocking narrow paths. Ezio is not damaged by caltrops.

Lamb’s Blood – Blood Bomb (Tactical) – Sprays blood in an explosion, causing fear and panic. They incapacitate, but for only a short amount of time.

Shrapnel – Splinter Bomb (Lethal) – Sprays shards of metal in all directions, hurting or killing anyone caught in the blast. Like a modern frag grenade, beware of killing civilians.

Salt of Petra – Smoke Decoy (Diversion) – Releases a thick smoke that draws guards to it. Useful for luring guards and breaking their line of sight. Less noisy and more discreet than other diversions.

Phosphorous – Smoke Screen (Tactical) – Blinds victims with a bright flash and creates smoke that breaks line of sight. Enemies caught in the smoke are easy to kill, and Ezio can see through the smoke with his eagle vision.

Datura – Datura Bomb (Lethal) – Releases a cloud of poison smoke that hurts and incapacities anyone caught by it. Beware of killing civilians, the effect is similar to the Poison Dart.

Coal Dust – Thunder Bomb (Lethal) – Creates a powerful explosion, it is less powerful than other bombs, but will stun and disable anyone not killed in the explosion.

Skunk Oil – Stink Bomb (Diversion) – Drenches anyone with a strong stink caught in the blast, very useful for separating targets from crowds or other guards. Any guard caught in the stink will stop until the effect has worn off.

Sulfur – Cherry Bomb (Diversion) – Pops with a loud noise, and attracts guards from far > and wide to investigate the noise.

Pyrite Coins – Gold Bomb (Diversion) – Like throwing coin on steriods, the explosion will spray an area with coins, any citizen will become hostile to guards coming to investigate, creating a big instant crowd and diversion.

Bomb Casings

Fuse Shell – The default casing, it bounces off surfaces or NPCs and explodes after three seconds. The extra time can be useful for escaping and hiding before the bomb goes off.

Impact Shell – Explodes on impact with an NPC or surface. If you need an immediate > explosion, use this casing. Best used for lethal bombs, where you’ll rarely need to set a killer trap, or just need to defeat a large group quickly.

Trip Wire – Explodes when enemies walk near, a perfect casing for setting traps. Set these on guard patrol routes.

Sticky Pouch – Sticky to any surface of NPC it lands on, then explodes after five seconds. The delay helps Ezio escape, and being able to land a bomb on a specific target means you always know at least one of your intended targets will be caught in the blast.

Bomb Gunpowder

Indian Gunpowder – 2 meter blast radius. Best used for precision attacks where you need to limit civilian casualties.

Arabic Gunpowder – 3 meter blast radius. Medium, for when you need a slightly larger effect on your targets.

British Gunpowder – 4 meter blast radius. Best used for large groups, save these large blasts for tactical bombs that limit civilian death.

I'm not really sure of how to word this nonconfusingly, so I'll just take a stab at it - just like your targets are color-coded in Eagle Vision, so are the places they've been. So, red trails mean a guard has been walking there (and probably will be again soon if you hang around) so this gives you some indication of the relative safety of a location in a broad sense.

Mobile mission targets, as well as captains, show up in gold - there are a few missions where this is pretty much the ONLY way to reasonably track down your target. (Two of the Master Assassin missions I've done so far, I think would be basically impossible without it - the first of these that I did tipped me off that I should follow the trails, though.)

This is easiest to observe in any mission where you have a companion who follows you around - they're gold, and you can see them leaving a gold trail behind as they walk around.