Caution needs to be taken when choosing grey literature sources. "Unlike commercially published materials, grey literature often does not go through a rigorous quality control and production cycle" before published. (Grey literature in Australian education)

Government information is often considered as accurate, official and authoritative; however, government agencies may present material in a way that reads favourably for the government. It is important to have some understanding of the author's stance on a particular topic or consult other resources for additional information and context.

NGOs can also be lobby groups for corporations.

We also need to be aware of the money's influence in politics. Think tanks are not-for-profit and produce independent and non-partisan research and analysis for policy issues. They get most of their money through donation by large businesses and are not required by law to list their donors. Check Transparify for funding transparency ratings for think tanks.

Check the Citation styles for the type of document you are working with, for example: web site, dissertation, technical report, government information and etc.