Is the JBL Stage GT50011 enough for a subwoofer?

It is a mono amp made for one subwoofer channel. Match it to a sub whose power handling fits the amp's rated output, and it will drive it cleanly.

Does it work with a factory head unit?

Yes, if the head unit has line-level or speaker-level outputs the amp accepts. Check the input options on the amp before buying.

Can I run two subwoofers on it?

Yes, if the subs are wired to present a load the amp is rated for. Two 4-ohm subs in parallel give a 2-ohm load, which is common for this type of amp.

Does it have a built-in crossover?

It is a mono subwoofer amp, so expect basic low-pass filtering rather than a full DSP. Confirm the filter controls on the unit before you buy.

Why choose Class D over Class AB?

Class D runs cooler and draws less current for the same output, which suits tight installs and stock electrical systems. Some purists prefer Class AB, but for subwoofer duty Class D is the practical choice.