It seems to me that the cause is the automatic way that gnuplot (the underlying program that gretl uses for plotting) calculates the number / frequency of the tic marks.

Indeed, if I add the line
set xtics 1

to the plotting commands then I get rid of the non-integer tics.

(To use this workaround, you have to right-click on your plot, choose "save to session as icon", then in the icon view window you'll have a "Plot 1" icon, right-click on it and choose "edit plot commands", and there you will see the code in gnuplot syntax in a new editor window. Add the line above, click on the save button, and then on the run/execute button.)

cheers

sven