The base movement speed of a gnoll flesh gnawer is 30 feet, and if it activates rampage by bringing a creature to 0 hit points, it can move an additional 15 feet for a total of 45 feet during "normal" combat.

But is there a way that rampage could be triggered for the flesh gnawer after it gains a movement speed of 90 feet from its Sudden Rush action? This should allow the flesh gnawer to traverse 135 feet in one turn.

Ideally this would require only actions/abilities possessed by the various gnoll types, but I am open to other first-party/non-homebrew solutions for Dungeons and Dragons 5e that may make this possible.

Potentially relevant text from some of the gnoll actions are listed below:

Sudden Rush

Until the end of the turn, the gnoll's speed increases by 60 feet and
doesn't provoke opportunity attacks.

Incite Rampage (Possessed by the Gnoll Pack Lord)

One creature the gnoll can see within 30 feet of it can use its
reaction to make a melee attack if it can hear the gnoll and has the
Rampage trait.

Rampage

When the gnoll reduces a creature to 0 hit points with a melee attack
on its turn, the gnoll can take a bonus action to move up to half its
speed and make a bite attack.

It is possible by cooperating with a Gnoll Pack Lord

A Gnoll Pack Lord needs to help. You also need a victim to be reduced to 0 hit points by the Flesh Gnawer for Rampage (hopefully a creature adjacent to the Flesh Gnawer). Here is how the situation can play out:

1. On the Gnoll Pack Lord's turn, it takes the Ready Action: it will use Incite Rampage on the Flesh Gnawer when it says "now" (or some other appropriate trigger).


2. On the Flesh Gnawer's turn, it takes the Sudden Rush Action, improving its speed to 90 ft.


3. Still on its turn, the Flesh Gnawer says "now", the Gnoll Pack Lord takes its readied action, allowing the Flesh Gnawer to make a single melee attack against an adjacent creature using its own reaction and reduce it to 0 hit points.


4. Since it is still the Flesh Gnawer's turn and it used a melee attack on its turn to reduce a creature to 0 hit points, it is eligible to activate Rampage as a bonus action and moving 45 ft.

Note that this requires a lot of action expenditure: the Pack Lord spends its Action on the Ready Action as well as its Reaction and the Flesh Gnawer expends its Action on Sudden Rush, Reaction on the Incite Rage provided reaction and bonus action on Rampage.

Using this process may also feel convoluted and even a bit of metagaming^1. on the DM's part. The gnoll pack lord needs to somehow know that the PC is possibly going to be dropped to 0 by the gnawer's next attack. It is also waiting specifically for the gnawer's turn instead of just inciting as part of its multiattack which is much more efficient in most circumstances.

^{1. Noted by NautArch}

When haste has been cast on it

A standard Gnoll Flesh Gnawer (Volo's, 154) has to choose to between each action option and does not naturally get more than one action.

In the normal scenario, the creature can either use their Multiattack or Sudden Rush. They can not normally do both.

If they've chosen Sudden Rush, then there is no action remaining to make a melee attack that would trigger the Rampage.

This is where haste(PHB, 250) comes into play. If they are under that spell, then they have an opportunity to make a single weapon attack. In this scenario, they could use their Sudden Rush as their primary action, and then make another weapon attack with their haste action. If they were to reduce a target to 0HP with that attack, then they'd be able to trigger the Rampage.

Not only that, but their base speed would be 60 rather than 30. The Sudden Rush would yield a speed of 120. And if they connected and triggered Rampage, then they'd get another 30(due to half their current now being 30, not 15.)