Complex sentence: He took the young boy in under his wing and began to mentor him for a short while, until it was discovered that not Sunspot but his New Mutants teammate and best friend, Samuel Guthrie (a.k.a. Cannonball) was the new External they had been seeking.
Split into simpler sentences: He took the young boy in under his wing and began to mentor him. Gideon mentored Sunspot for a short while, until it was discovered that not Sunspot but his New Mutants teammate and best friend, Samuel Guthrie (a.k.a. Cannonball) was the new External they had been seeking.