Transit agencies will often release GTFS data under a license that governs the terms of its use, including how it may be utilized or displayed. If an agency publishes a license with its GTFS data, it can typically be found under their "Developer Resources" or similar page. Sometimes, agencies will not include a formal legal license, but instead will provide guidelines for how the data should or shouldn't be used. For more on GTFS licensing and data dissemination, visit TransitWiki.
The National Transit Map requires all participating agencies to agree to license their data through the Creative Commons CC-BY-3.0 license.
Your agency can control access to the data by using and issuing API keys to users of the data. If your agency does not currently host its static GTFS files on its website, contact your webmaster or IT specialist - they should be able to set up a simple hosting URL for you.
For static GTFS data, requiring potential users of the data to have an API key is not considered to be a Best Practice. Requiring an API key substantially limits the potential user base, and makes it more difficult for developers and entities such as the National Transit Map and Transitland to consistently access your data.