Or, just make a new sub-race:
As a gnome touched by the nine hells, you have a natural cunning and cruelty beyond that of other gnomes.
Ability Score Increase. Your Charisma score increases by 1.
Hellish Resistance. You have resistance to fire damage.
Infernal Legacy. You know the thaumaturgy cantrip. Once you reach 3rd level, you can cast the detect thoughts spell once per day. Once you reach 5th level, you can also cast the suggestion spell once per day. Charisma is your spellcasting ability for these spells.