AT-A GLANCE oil tank gauge

I have an old AT-A-GLANCE OIL TANK GAUGE that isn't registering. I wanted to replace it but when I unscrewed it the glas housing came off only. the threaded portion with the float attached remaim firmly on top of the tank. Any suggestions?

Yep. You have to totally remove the gauge from the tank. It's all attached underneath.

How do I do that??