looks like the rubbber stopper on the gauge wasnt fully seated and the liquid in the gauge leaked out. Is it trash or can i fill it with something else comparable?

unless youre really good with aking apart tiny things, its junk

Don't bother...liquid filled gauges that are exposed to heat don't work well anyway. Get a non liquid filled and be done.

the liquid just keeps the needle from bouncing. refill it if you can.

You are correct but heat affects it's calibration. Ask me how I know.