So no need to say how useful it can be with the near one hundred layers we can have with the Custom Font, but you should know that very few filters have a different behavior with that "input -> all" and in my very first custom font I did encounter this "behaviour".
How I did use G'MIc for my first custom font and which effect I did used?
I wanted a 3D map, thus I used a 50% gray for generating the custom font.
Then I did Filters/G'MIC/Rendering/3D lathing on the letter B (this letter is very important on this particular filter to see if the result will not be flipped horizontally!)
Tip increase the resolution at last as this takes a lot of resources and slows the preview and other setting. .
EDIT: The quote below is no more accurate, G'MIC updated its filters, fixed the re-naming and the "texture above layer" (now unchecked by default )
So just "input all" before clicking OK, and it will be done in 1 click without the need to rename
Then to put my texture on each layers, I put it at the bottom of the stack in mode "Multiply" or "Linear Burn" >> thus no need for mask (use opposite respective mode if your texture is darker than 50% gray)Sadly this filter cannot be used for the all layers input at the same times for 2 reasons, as noted, if there is a layer above it will be used as a texture ( I learned it the hard way as I never read the "notes") .
so the first time I used it for ALL layers, the filters used letter A to "texture" letter B, C to texture letter D and so... and it's a very resources demanding filter, I've waited something like 15mn to render all the custom fonts at once...
The second thing is that this filter rename the layer as "[3D lathing]" .
So what I did is to made one layer with G'MIC (all other layer are NON visible!), then select another layer > make it visible, remove visibility of previous layer then Ctrl+F on the visible layer (Ctrl+F repeats the last filter used), then select another layer > make it visible, remove visibility of previous layer then Ctrl+F on the visible layer, then select another layer > make it visible, remove visibility of previous layer then Ctrl+F on the visible layer, then ... yeah all characters like that (count ~15 to 20mn to do all), then renaming properly each layer (I did a Ctrl+C on ".png" then double click on the layer name, input the right character + Ctrl+V, then next, then next.... ~5mn of boring work.
Before anything save the "space.png" layer to another image (just drag that layer on the toolbox) it will open as new image. >> and come back to your Custom font.
Then I used this script "Filters/Animation/Overlay Background" to duplicate the BG over all layer in 1 click (Shift +Click on an eye in the layer stack to make them all visible except the "space.png" before running the script)
To download it > http://chiselapp.com/user/saulgoode/rep ... combine-bg
Your text is now textured, if the layer "space.png" was affected just drag n drop it from the other image (did you "saved" it?)
Now Second G'MIC's effect (plastic-ish effect)
This one is very easy as the input ALL is working smoothly and fill all layers in 1 click
Filters/G'MIC/Light and Shadows/Relief Light >> Input Layer > ALL > try out the few sliders if you wish, but the default setting are already good enough, then Click OK
Then I cropped the image (all layer at once) to compensate the 3D lathing centering
This line is just used to ping @Alan I hope that I did the right screenshot he is interested with