Gem stalkers are odd, and that’s not just my bias against gem dragons talking. They’re kind of a jumble, even for unevolved creatures—which they are, being created by gem dragons out of the leftover organic material of liquidated aberrations.
Let’s start with their ability contour: highest in Strength, with high Dexterity, Intelligence and Constitution to go with it. Their Dex is a smidge higher than their Con, although the modifier is the same, so while there’s a slight bias toward shock attacks over brute melee fighting here, they can flex in a pinch. Proficiency in Perception and Stealth indicates a predilection for ambush, harmonious with the shock attacker role, and their climbing ability (climbing speed plus Spider Climb) fits into this schema nicely as well.
But then we get into the strange stuff. Their Multiattack is savage: four Claw attacks, each of which deals an average 10 damage. Melee, yes, please! Shock attacker and brute are both melee-intensive roles. What’s this, though? A bonus action ranged spell attack, Crystal Dart? That’s not useful in melee. If a gem stalker is engaged in close combat with a target, it’s going to have disadvantage on ranged attacks—all ranged attacks, whether against the same target or a different one. This bonus action clashes with the gem stalker’s melee Multiattack, suggesting that it’s always used before the gem stalker closes and engages. (There’s also the fact that Crystal Dart includes one of five different riders, depending on the type of gem dragon that created the gem stalker, but more on that in a moment.)