The file appears to be a specific development or community-distributed build of s&box (pronounced "sandbox"), the spiritual successor to Garry's Mod developed by Facepunch Studios . Project Overview
Large build IDs often correspond to specific depot updates on SteamDB. S&Box.Build.10376186.zip
Zip files with these naming conventions are frequently shared in developer communities or via "giveaway" systems before the official Steam release. The file appears to be a specific development