I saw that graphic, but it wasn't clear to me (I do better with written instructions, rather than visual). So you don't put any descaler in the oven itself? Just soak that piece?

Please feel free to conatc me as well directly so I can assisst you.

Here what the manual says:
"To keep your appliance in good working order, you
have to descale the side plate regularly, if no Brita
descaling sytem WF 040-020 has been installed in
the water supply.
Descale the oven when the descaling symbol n
appears on the display.
Leave the oven to cool down completely before
descaling!
Be careful not to scratch the oven interior when
removing the side plate. Small parts might fall into
the base strainer. Tip: put a kitchen cloth over the
oven base.
Only use the descaler recommended by our aftersales
service (order number 311 138). Other
descalers may damage the appliance. Mix a
descaling solution of 500 ml of water and 100 ml of
descaler. Do not let the descaler come into contact
with the frame of the oven.
Do not use any scouring or abrasive cleaning
agents to clean the side plate.
How to Proceed
1 Pull out the left rack after undoing the knurled
nut on the front side.
2 Detach the grease filter in the upward direction.
3 Undo the forward bolt with a socket spanner
(13 mm). Depending on the version of
appliance, also undo the rear bolt with a socket
spanner (13 mm). Remove the side plate and
descale it.
4 If neccessary remove scale build-up inside the
oven compartment with descaling solution.
Rinse thoroughly any descaling solution with
clear water.
5 Re-insert the side plate. Re-attach the grease
filter and the racks.
6 Check that all parts are fitted securely .
7 Reset the descaling symbol in the basic
settings.

Here is a link that might be useful: http://www.gaggenau.com/us/service/downloads/index.html

I did the descaling with Dugol and reset the symbol, but the descale symbol keeps reappearing, though when I took the unit apart again the parts all seemed very clean. Our unit has never looked even slightly dirty, though there was some stuff to soak off on the inner fan blade when I did so last month.

The symbol comes and goes; it's not there all the time.

Also, I THINK I reset the symbol by choosing the option, but maybe I'm doing it wrong. Do you just select the "reset" option and then push the back arrow to get out of the menu? I didn't get any confirmation of the rest (beep, change in the visual) and I'm just wondering if maybe that's the issue.

I've read there is a filter you can add to help with hard water. While we never had an issue in our first five years of use, can we add it now? Should we?

We've got an electronic descaler on our water system, but not a water softener. We have hard water, but not egregiously so. We did set the water hardness, with a test strip, when the unit was installed.

Anyone have any advice/similar experiences?

Bumping as this is still occurring. The oven is working fine, but the descaling symbol comes on anytime it's in use. I've descaled twice, and reset warning multiple times. It's still happening. The first four years it never came on at all. Worried and frustrated.

Bumping, 5 years later! We have a Gagg 270 series and the instruction to just clean a plate seems wierd. Anyone have recs on how to descaler an older plumbed steam oven without a cleaning function?