Replacing t-stat
 

Had the heat on full blast and was working fine while moving, but when I came to a stop, it was much colder. Figured it's a thermostat going bad.

http://dz310nzuyimx0.cloudfront.net/...f8ded59f89.png

Is it necessary to replace the gasket as well? There doesn't seem to be a part number in the above listed diagram, unless it comes with it already. If not, maybe liquid gasket will work?

Liquid gasket would work fine. You could also buy gasket material and make your own. Guaranteed there is a part number for it. Call a dealership or Lordco and track one down.

Why do people asking for mechanical advice, insist on not posting the application?

Check on a specific forum to get an idea of what they're doing. If it has a bypass hole, make sure it's clocked correctly.

A reverse search on the photo shows it's for a Toyota Matrix/Pontiac Vibe, but for all we know, it could be a BMW. Because cooling system problems. :troll:

In that case, there should be a rubber seal that the thermostat slips into, before installing it into the housing. If it's not included, it should be available, separately.