How to get it

Go to https://steamcommunity.com/dev/apikey to request or revoke your key.

Environmental variables

Set your key as the value of the STEAM_WEB_API_KEY env var to enable steam API features.

GSW Control Panel options

Configure the steam app ID field for a game server to enable a backup lookup for the server via the steam API if the gamedig query fails.
The app ID for games can be found here: https://steamdb.info/apps/

Note: The steam API query doesn't return individual player info, only player numbers.