Gel nails is a viscosity like gel product that is used to build artificial nails.  It is applied on a prepared nail bed and cured under U.V. light. The result is a strong, flexible nail with a high glossy finish.  It comes in many different color and should last until it is pushed out by the natural growth of the nail.

At the base, acrylic was used to build: that is the reason for the strong fume.  A top layer of gel was used to finish up: that explains the use of UV light for curing.  Some of the reasons Acrylic was used to build the base are: either the tech did not know how to use Gel to build or acrylic was used for some cost savings measures.

Instance one: Asked for Gel but told to use "powder gel":
When gel is in powder form, it is acrylic !  Gel must be in viscosity form like thick honey. 
Instance two: acrylic is used at the bottom and gel is applied as a top coat:  This is a shortcut where the technician does not know how to build using gel and use acrylic to build . You most likely got charged as gel  price.

Gel nails if not applied correctly, are easier to lift: if it's not happening within 7 days, it's not going to happen.  The other issue is that you might be allergic to gel: you'll know within an hour or two.