You could add a dummy series consisting of two points so that the line
joining those points lies over the top of the particular gridline you
are wanting to draw attention to.

I'm assuming your chart is XY (Scatter). Say you wanted the horizontal
gridline at Y = 10 to be red and thicker and that the maximum value on
the X axis is 50. After selecting the chart you could go Chart|Source
Data... to bring up the Source Data dialog. Click the Series tab then
click the Add button to add a new series.In the X Values: box type
={0,50}
In the Y Values: box type ={10,10}
Then click OK.
On the chart , select that new series and format it so that there are
no markers and the line is thicker and red (assuming your other
gridlines aren't already the thickest).

If the X axis maximum value is Auto you will need to set it to 50 so
that the dummy series line goes right to the edge of the chart.

Ken Johnson