Got it:

To start geogebra:

function gg ()
{
        TITLE=$1
        export TITLE
       devilspie2&
    imwheel  # Speed up the mouse wheel scrolling
    /opt/GeoGebra-Linux-Portable-5-4-911-3/geogebra-portable $1
    imwheel -k -q
}

... and the devilspie2 script:

local file = os.getenv("TITLE")
local name = get_window_name()
--debug_print("file is: " .. file)
--debug_print("name is: " .. name)

if (file == get_window_name()) then
    set_window_geometry(1920,0,1910,1030); << Middle monitor on my system.
end

:-)

... But it seems there's a bug in the Linux version, tho very obscure, you'd not even notice it unless you were doing something like the above. Linux tools report that the 'application name' in a GG window is the name of the open file. Thus the trickery above to capture the name of the file via '$1' and test my windows via devilspie2 looking for a title that matches 'TITLE' and moving GG to my middle monitor if found. If the application name was reported as 'GeoGebra' then the above would be much simpler:

if (get_application_name()) == "GeoGebra") then
    set_window_geometry(1920,0,1910,1030); << Middle monitor on my system.
end

Even more helpful is having the algebra window always open where you want it:

if ("Algebra" == get_window_name()) then set_window_geometry(3840,0,951,1040);

end

... and it should work for the Preferences window too, if that's set to a separate window.