The guild leader can increase the size of the guild fief by using "Increase the fief" option on the guild monument or on an outpost.
To successfully increase fief size - the leader must have 10 silver ingots and 5 gold ingots in their inventory.
Additionally - Fief must have either Guild monument on it or Fief must neighbor a Fief that has Guild Monument on it.
Increasing fief size causes borders of the fief to expand by one tile.
If you to expand neighboring fief same way - you will be able to connect fiefs and eliminate strip of neutral land between them.