I want to know can I use Pyrex glass round casserole as a safe cookware on gas burner?
Does this cookware release toxic chemical in high temp?
I only worried about toxic compounds and not about cracking!
Update based on edited question: there are no issues of toxicity.
It is a very poor idea to use glass cookware on a burner. Not all Pyrex is made from high quality borosilicate glass anymore, and even if you have some, the issue is thermal shock, not toxicity.
If you heat or cool glass very rapidly, the internal stress caused by thermal expansion (or contraction, when cooling) can cause it to shatter. This can be a real safety issue when it is full of hot food.
For this reason, you generally only want to use glass cookware in the oven, and never transfer it directly from the freezer to the oven, or the oven to the freezer.
The symbols for that glassware do not indicate stove top use.
I have owned glassware (Corning Ware Visions) that was specifically build for stove top use, the design, glass color and symbols designation were different. The 300 degree centigrade rating would be on the low side for a gas flame (900 - 1500 deg C).
I also have personally had oven-safe cookware fail when using on an electric hob. It crumbled after a while and contents went everywhere, so I definitely wouldn't risk it.