Just to be technically accurate - Simtec or Prestec are not gel
coats... they are polyester topcoats. Gel coat is formulated as an in-
mold coating, but can be used as a post-mold coating with the addition
of a "air-dry" additive. Linear polyurethane (i.e., two-part mixture)
is a coating formulated for finished spray application. Simtec is
essentially a polyester-based finish coating designed for application
similar to a polyurethane paint, except using polyester chemistry.

So we have:

Gel Coat - Designed as an in-mold coating, but can be modified for
application as a spray finish. Requires significant finish sanding and
polishing.
Topcoats - Designed as a finish coating - Can be urethane or polyester
chemistry. Depending on quality of spray application, finishing can
range from no sanding/polishing to minimal post-finishing required.

Hope this helps.

Bob