"I tried!" Gabriel says. "She didn't budge. Besides, she's no danger." As if to prove it, Gabriel lifts Summers out. She's the size of a compact loaf of bread, with six legs that end in soft paws that tread the air. She twists her neck until she can see Gabriel, at which point she relaxes and purrs. "I looked up her species in an extra-dimensional catalog. The guy who first summoned them wrote that they're peaceful. They can create their own dimensional rifts. As best the practitioner could tell, they use them to hide from predators. Occasionally they mistake one of our rifts for one of theirs."
You've worked closely enough with Gabriel to realize that he's lonely. He hasn't had any real friends among the pattern magic students, and ignored socializing for research last fall. That's a large reason why he wants to keep Summers.
If he keeps her and is found out, then it won't be just him who gets in trouble. You'll be implicated as well. Plus, Summers could distract him from research.