Most manicure courses will not cover gel polish application, you will need a separate course.
Many will say that if you can apply polish, you can apply gel and so don't need to actually do a course as long as you have a mani certificate, and indeed a large percentage of gel polish suppliers only require a manicure certificate for you to purchase their products.
I would always recommend a course, so you fully understand the use of your products and chemicals; but what it will ultimately come down to is your own confidence in your skills, but more importantly, your insurance company.
Some will be happy to cover you to provide gel manicures without a specific class, only manicure, others specify that you must have a certificate in gel polish application to be insured for that service.