As a Central Texan, I don't come across a typewriter like this one everyday.  In fact, this is the first I had ever heard of a Gossen-Tippa-B typewriter.  I looked it up, and I think Adler had something to do with them sometime later.  And as for that knob, I am completely unsure of just what it does.  About the only thing I can recommend is to first, warm up some automatic transmission fluid (not to boiling--just warm), set the typewriter in a pan or something that will contain oil, and brush plenty of the fluid in, and let it soak overnight.  The next day, exercise the keys until they are loose, and everything else is loose.  Keep exercising all the joints, but don't use too much force--don't want to break something.  Eventually, one at a time, the keys will come unstuck, and maybe that knob.  When that happens, follow the linkages that come from the knob to see where they end up and what they make the typewriter do (or not do).  Let me know, and let me know how you come out in this deal.  Oh, and by the way, when everything is unstuck, wipe out all the excess fluid.

Sometimes the typebars will be fine, but all those typebars are directly connected to the linkages below, which may be the ones sticking.  Let's say that for every type bar, there are three or four links, counting the connecting rods.  If just one link is stuck from the key to the typebar, the whole works will be stiff.  The link may be stuck in the position that it has the type bar slightly pulled up, but won't come down.  I have come across HUNDREDS of typewriters in this condition.  Not to worry!!  That good ol' tranny fluid should loosen up the works and everything will lie down nice and flat for you, even after you've used the typewriter.

xntryk1 wrote:

I guess I stumped everybody...  

Patience...

 The forum isn't a 24/7 tech support hotline and sometimes it can take a few days before someone will respond to a repair question such as yours.

xntryk1 wrote:

...after pushing them down (hard). Also, most of the hammers refuse to lie down.

It's very possible that the linkages between the type and keybars have been bent from being forced. Typewriters are precision instruments and as such you shouldn't really force anything, or use most effort than you would to normally type with it. I'd suggest cleaning everything thoroughly so that all the parts are moving freely before addressing the repose of the typebars.

xntryk1 wrote:

Thought about just dousing everything in WD-40. 

Never use WD-40 in a typewriter. Ever. There are exceptions to this rule, but that's not what you're dealing with right now. And forget the transmission fluid too. All you need is a bottle of mineral spirits and a box of cotton swabs (Q-Tips) to clean the pivot points for the typebars and keybars.

xntryk1 wrote:

Also, there's a little knob on the left-hand side of the keyboard. It's marked with the letters "S" and "I" (or lower case "L") with arrows pointing to each letter. I presume this knob should turn to each letter when you twist it, but it doesn't. It seems locked in place.

The knob is a touch control adjustment. You turn it clockwise (s = schwer or heavy) to make the type feel heavier and counterclockwise (l = leicht or light) to make it feel lighter. I wouldn't worry about its operation until you have cleaned the machine and it's typing normally.

xntryk1 wrote:

I have NO idea where "the pivot points for the typebars and keybars" might be located. Looking at the picture of the bottom of the typewriter, would that be somewhere behind the bottom plate?

A good place to start is at the segment (see photo below), the large metal plate beneath the ribbon where the typebars pivot on their way to the type guide. Soak a cotton swab in mineral spirits and work it into those slots and clean the section of each typebar that disappears into those slots. Be liberal with the spirits, and repeat as often as is required. Slowly lift and lower the typebar by hand to check its movement and verify that it's moving freely. Doing this should have a profound effect on your machine's operation if the root of the problem is dirt, which is very often is.