There are other primary sources, but all are later than, and at least partially reliant on, Caesar's Commentaries.

Plutarch's Life of Caesar (15-27) summarizes the Gallic Wars. Suetonius' Life (22-5) does the same, very briefly. There is a much more substantial account in Books 38-40 of Cassius Dio's Roman History - but be advised that Dio was writing almost three centuries later, and has a habit (like many ancient historians) of inventing speeches for his characters.