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:
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.
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.