yes, you can... but I would buy the foam sleeves and use them anyway.
The foam sleeves are so much easier to use and they are cheap to buy. They will provide a good air seal that you may not get with the fiberglass. I bevel the corners to fit around elbows and it results in a nicely fitted installation.

I use zip ties where the insulation ends to close up the gap around the insulation and the pipe. I also use zip ties anywhere the adhesive needs a little extra holding power.

Here is a link that might be useful: foam pipe insulation

Ok. Is any brand better than others?

Can I take the unfaced insulation and wrap it around the gas water heater?

Thanks.