How do I fix these gaps where the socket plates don't cover?
The best approach is to fix the plaster and paint. Some other ideas:


Install oversize device plates. Since you have nice stainless-steel plates now, this will be more expensive, but it would be quick and easy.
You may be able to find or make backing plates that fit around the devices, behind the existing plates, and extend outward a small distance. 
As a last resort, fill with spackle or painter's caulk, then touch up the paint.