How to ...
... include gd pictures in TeX
Please consult the section on printing
... place legends
You may use the command GDESCR to place them interactively
or statically with the line tag GDESCR: in the input file.
Both methods offer legends with frames with their FRAME command parameter.
... plot functions with gd
Arithmetic operations with the data columns in the input file
allow 1-dimensional functions to be plotted.
... make thicker lines
Use the display parameter CMODE() to make thick characters.
Use the display parameter LINEWIDTH() to increase the base linewidth.
Use the T linemode in the data block headers of the input file to specify lines of multiple thickness.
... make larger symbols
Use the display parameter SYMBOL() to increase the symbol size.
... make larger axis descriptions
Use the display parameter AXFAC() to enlarge the axis description.
Often AXFAC(1.5) already gives acceptable results, in particular for overhead slices.
... produce picture overlays
There is a whole chapter on overlays describing this issue.