If you liked "Gregor the Overlander"...

Thank you for your Book Match request. Gregor the Overlander is a great fantasy series! Thank you for telling us why you really liked it, this really helps us understand what kind of books you are looking for! Here are some other books that you may enjoy:

The Marvelous Effect (Marvelous World, book 1) by Troy Cle.
Strange things are happening on Earth, and twelve-year-old Louis Proof discovers that he is one of the few people able to see--and combat--the responsible beings, who came from Midlandia, the planet at the center of the universe, to continue a battle that rages there.

Artemis Fowl by Eoin Colfer.
When a twelve-year-old evil genius tries to restore his family fortune by capturing a fairy and demanding a ransom in gold, the fairies fight back with magic, technology, and a particularly nasty troll.

The Amulet of Samarkand (Bartimaeus Trilogy, book 1) by Jonathan Stroud.
Nathaniel, a magician's apprentice, summons up the djinni Bartimaeus and instructs him to steal the Amulet of Samarkand from the powerful
magician Simon Lovelace.

The Cryptid Hunters by Roland Smith.
Twins, Grace and Marty, along with a mysterious uncle, are dropped into the middle of the Congolese jungle in search of their missing
photojournalist parents.

The Dark is Rising by Susan Cooper.
On his eleventh birthday Will Stanton discovers that he is the last of the Old Ones, destined to seek the six magical Signs that will enable the Old Ones to triumph over the evil forces of the Dark. This book is now a movie called "The Seeker".

So You Want to be a Wizard by Diane Duane.
Thirteen-year-old Nita, tormented by a gang of bullies because she won't fight back, finds the help she needs in a library book on wizardry which guides her into another dimension.