Why This Pair
First, Gohan and Loki have some things in common.
They are both outliers in their universes. They have both been alone and different from everyone else. They didn't really have any of their own friends. Gohan's first friends were his Dad's friends, and Loki most hung out with Thor's friends. They also do not enjoy fighting like those around them, with the exception of their mothers, who can fight but choose not to. Even though they do not love fighting, they are good at it. They are also both smart and scholars. They are both royalty and come from warrior societies.
The Sons tend to change characters by lightening their darkness, so Gohan would probably bring out the best in Loki, whether Loki was a villain or not.
Loki could help Gohan be comfortable with himself as an individual, as well as with the power inside of him.
Gohan and Loki wouldn't have to worry about hurting each other since they are stronger than the average human.
They would also be comfortable with each other's loved ones.