Ability duration is longer, and MOAB-Class Bloons spawn with less health during the sabotage.
~ BTD6 description
Grand Saboteur (sometimes referred as Grand Sabo, or shortened to just "gsabo") is the final upgrade of Path 2 for the Ninja Monkey in BTD6. It improves its own Bloon Sabotage ability that additionally causes all incoming MOAB-Class Bloons to spawn with only 75% of their original health before entering the track, along with the usual 50% slowdown to all bloons on screen. In addition, the ability (including the duration for inflicting partial damage to all incoming MOAB-class Bloons) lasts for 30 seconds instead of 15 while maintaining a cooldown of 60 seconds.
Grand Saboteur further improves the Bloon Sabotage ability to last twice as long and inflict instant 25% damage to all incoming blimps on screen. Although considerably more expensive than multiple Bloon Sabotage abilities, the instant percentage damage and increased duration can come in handy in specific scenarios. Outside of certain challenges, it is mostly used in super high late-game runs to deal massive damage to incoming BADs.
The Grand Saboteur is very useful on Rounds 80+ because MOAB-class health scales from that point due to Freeplay Rules, for modes including Impoppable and CHIMPS, which both require Round 100 to be beaten.
The enhanced slowdown along with the weakening of all new MOAB-Class Bloons makes it very powerful against MOAB-Class Bloon rushes and especially against D.D.Ts.
Since Version 3.0, which from then on both Bloon Sabotage and Grand Saboteur can carry over rounds, it is possible to damage a BAD immediately before it appears by activating the Grand Saboteur ability right before Round 99 ends.
This, combined with Ground Zero or Tsar Bomba, can deal with round 95 if the track is long enough. This is a recommended strategy for CHIMPS.
The blimp health reduced from this tower's ability will add to the damage count of the tower. For example, the B.A.D. at Round 100 has 28,000 health, and when the ability is used at the end of Round 99, the B.A.D. will spawn with 21,000 HP, and the lost 7,000 HP will be added into this tower's damage count.