Is 300 ANSI lumens bright enough?

Only in a dark or heavily dimmed room. With lights on or daylight, the image will look washed out. Plan on blackout curtains for the best picture.

How long does the laser light source last?

Laser sources typically run tens of thousands of hours before dimming noticeably, far longer than lamp projectors. You should not need to replace it during normal use.

Does it support 4K HDR streaming?

Yes. It has a 4K panel and HDR support, and Wi-Fi lets it stream from apps. Check that your streaming device or the built-in apps output HDR.

What screen size can it throw?

It depends on throw distance and the lens; a 4K projector like this is typically used for 80 to 120 inch screens. Confirm the throw ratio for your room before mounting.

Is it good for gaming?

Only in a dark room. The dim output and unknown input lag make it a better fit for movies than fast-paced games; check the lag spec if gaming matters.