I'm an editor for a beauty magazine. Gel and shellac are not the same thing. Shellac is what is known as a gel polish, it is applied like a polish and cured for a few seconds under a Uv lamp. It lasts about 2 weeks.
Gels require a filing of the nail plate. Layers of gel are applied and cured for a couple of minutes. These are stronger and can last 3-4 weeks.
Acrylics are even stronger. A mixture of liquid and powder is applied to the nails in layers. These can sometimes be thicker and also require filing of the nail plate.
Applied and removed properly, none of them will damage the nail.