If you're talking GSA Schedule or GSA Contract, the following would apply:

To get a GSA Contract (aka GSA Schedule, GSA MAS, GSA FSS), you prepare a proposal in response to one of GSA's solicitations. So it is not an application process, it is a proposal process.

Therefore, the direct answer to your question is that you submit an acceptable GSA contract proposal in response to one of GSA's Multiple Award Schedule (MAS) Solicitations. There is also some good training provided by GSA at their Vendor Support Center.

Preparing an acceptable proposal can be a difficult undertaking for those companies which have not responded to many federal proposals in the past. There are many components to the proposal and sometimes the instructions provided in the solicitation are not concise nor located in one place.

There are some books which can help you at Amazon.com and lulu.com; however, many of those books provide more GSA contracting theory than proposal preparation assistance. I know of at least one which provides six direct steps to a gsa proposal, but suggest you review all the possibilities.

Additionally, there are a multitude of companies which help businesses prepare acceptable test gsa proposals. The pricing for a completely written proposal ranges from a couple of thousand dollars to well over ten thousand dollars. You can also find consulting, webinars and training modules which are offered by some of these companies.

The proposal writing process can take up to 100-200 man hours, and GSA takes about 120 days to process proposals.