Well, I'm trying to add those auxiliar vertical and horizontal lines to my chart (grid), but I'm not sure if that's only a Highchart feature or it can be made with GWTHighchart too. P.S.: That picture is from a question (not mine) posted here at SO, but it shows what I'm trying to do.
Yes, you can do this. See the Axis api setGridLineXXX methods, eg http://www.moxiegroup.com/moxieapps/gwt-highcharts/apidocs/org/moxieapps/gwt/highcharts/client/Axis.html#setGridLineWidth(java.lang.Number)
Also, if the specific feature you want to access is not exposed by GWTHighchart then you may be able to use JSNI methods to access it.