A general sales agent is a sales representative for an airline in a specific country or region. Typically, the GSA is responsible for selling all products of the airline in its region which includes flight tickets and cargo space. A GSA will typically sell the product of more than one airline.
Airlines normally use a GSA in areas that it does not operate to or from, allowing them to have a sales presence in a country at lower cost than opening their own offices. It may also use their services because the GSA has historical ties with travel and cargo agents which will be too time-consuming for the airline to build itself.

The GSA receives a commission

Commission (remuneration)

The payment of commission as remuneration for services rendered or products sold is a common way to reward sales people. Payments often will be calculated on the basis of a percentage of the goods sold...

of around 3–5% on all tickets and unit of cargo space sold in the region that it represents.

All costs related to running the GSA's business are the responsibility of the GSA including but not limited to insurance, rent, general office expenses and any travel within the country or region needed to promote /sell the product.