Being able to talk to rats in a urban campaign is about as useful as that ability gets IMO. Rats are everywhere and can get anywhere.
The Rock Gnome frankly sucks though. Tinker is flavor at best and generally bloody useless, and Artificer's Lore is neat, but really, really situational. I'll take critter talk and the cantrip every time. The Rock Gnome's total package isn't as good as either of the abilities forest gnomes get. I'm not completely married to optimization, and I have no problem playing a sub-optimal Race/class pairing if it fits the character I have in mind, but I don't think I could pull the trigger on Rock Gnome regardless of concept. That makes me sad, because I like Rock Gnomes.
Honestly, give them the mending cantrip, leave the critter talk in, plus the CON vs DEX and I'm pretty happy. Maybe give them advantage or expertise with Tinker's Tools, or even Artisan Tools generally or a subset of that list (at the expense of critter talk depending on breath of application).