REPLACING CB400 CYLINDER HEAD GASKET

Is it possible to remove the Cylinder Head without disturbing the
Timing Chain and its Gear Wheel?

No

You don't have to break the chain just make sure it don't drop into the engine...I like to use welding/brazing rod.

Get a Haynes manual... Can get them for a fiver on ebay etc

I'd post one more post and you can get the manual for free on here..
But I like to read the manual before doing the job so that I can root out the tools and buy the right replacement parts, being a topend gasket set in this case. Good Idea to give it and oil and filter change as well.