Since you were specifically asking about gplot, here's how to do #1 in gplot:
proc gplot data=sashelp.class;
plot height*weight / vzero;
run;
Per #2, I don't know of a way to do that. There is a request for this feature in the defects/request system, and it might help bump up the priority of adding that feature if customers call Tech Support showing their interest, and providing specific examples of the things they'd like to be able to do with this feature. Here is the defect/request number to refer to:
S0134846: PROVIDE THE ABILITY TO SPECIFY JUST START, END, OR INCREMENT ON AXIS ORDER=