Glacier bay gal toilet.

For a half flush, push down handle and release. For a full flush, push down handle and hold then release.

ensure that the supply line is clear. shut angle stop valve off and remove line from bottom of tank place line into a pail and open angle stop valve, if clear water will flow quite readily. if not replace line. the fill valve is available at home depot. personally I would replace toilet with a quality American Standard. if your toilet is not a Glacier Bay good luck finding repair parts.

https://www.homedepot.com/p/Fluidmaster-Flapperless-Toilet-Fill-Valve-for-Glacier-Bay-Toilets-703AP4/202510398

Rona and Home Depot both sell Glacier Bay replacement parts.

A picture would be good. Glacier Bay might be the maker but the tank hardware could be someone else.

Glacier Bay Toilets are made in China by "various manufacturers" a local plumbing parts supplier is your best bet.

There should be a adjustment screw at the top of the fill valve, screw it up or down to adjust the tank level.

I have a glacier bay dual flush toilet. I have had it since 2009. When it started the hissing noise I tried diff things to fix it. New water valve, new washer from tank to bowel.
The food die in the tank showed water was leaking into the bowl. As a result the water would try to keep filling the take every time a little water escaped making it hiss. Sound familiar?
Finally I contacted someone on line home depot and the glacier bay manufacture was the main people. You need a silicone washer that goes in the main center piece mechanism that lets the water into your tank. You need to remove the center main piece in your tank. at the bottom inside the mechanism itself is a washer the size of a coffee mug top circle.
You can only see it if you take apart the two pieces. Once this goes inside your problem WILL BE SOLVED. The manufacture sent it to me for free. She knew right way. What causes the problem is cleaner tablets in the tank or bad water. So that's it folks

Try adjusting the water level down about a half inch or so and see if that helps. It could just be siphoning. Otherwise the valve may be defective. Hope this helps.