Dig in the zip file, there is three examples source files.

Oh I see that's awesome ! +1 for full 4-lvl grayscale tilemapping engine !

Well, I made one too. But it's dead as of now.

Yeah, as long as Runer wants to do something  in Axe, you can't do better...

I... guess so ?

How does one write text with the small font to the screen using GrayLib?

Assuming that you actually mean writing text to the buffers (since direct writing to the screen with the interrupt running would be promptly overwritten), you want to use included gray text printing commands. Set the °GIT (GrayLib Include Text) constant to nonzero (preferably 1) to include the string printing core routines, GStr(), GStrG(), GChar() and GCharG(), and optionally set °GIDEC (GrayLib Include DEC) to include the GDec() and GDecG() routines and/or set °GITOK (GrayLib Include TOK) to include the GTok() and GTokG() routines. Each routine expects the value of the appropriate type to be printed in r₁ (or simply given if the routine is a G...G() routine) and the color code to be in r₂. One slight deviation to this rule is that the token printing commands expect a pointer to the token to be printed.

Because color codes were a structure entirely of my own design, I should probably explain them a bit. The commented version does mention that a color code is of the form 0b0000FFTT, where FF is the fill color and TT is the text color. To clarify that a bit, that means that fill color is a value from 0-3 that represents the desired background color (since text printing clears the area behind it), text color is also a value from 0-3, and the two should be combined into one number of the form fill_color*4+text_color. And to be super clear, the values 0-3 represent increasingly dark colors. When using 3-level grayscale, 0 is white, 1 is gray, and both 2 and 3 are black. When using 4-level grayscale, 0 is white, 1 is light gray, 2 is dark gray, and 3 is black.

EDIT: Is it just me or is archive/unarchive broken with GrayLib on?

It's possible, I'm not really sure how well those commands handle a third-party interrupt running when they're called. It may just be necessary to temporarily disable grayscale when performing them. Simply using FnOff followed by FnOn may work well enough, but if not, use the full-blown GOff() followed by GOn().

Didn't you mention that Graylib used L4 which is used by Archive/Unarchive commands ?

Well...

Some minor issue I noticed with Graylib when setting up grayscale frequency in Reuben Quest: If the frequency is set too low on the 84+, the calc keys becomes unresponsive and you have to remove a battery.

Yeah that's what I thought actually. Unfortunately, however, for some calcs this might actually be the best-looking grayscale setting, so removing that particular setting might hurt quality for some people. Maybe there could be something like a timer based on CPU usage where if you didn't press any key for a certain amount of time it exits automatically?