To change individual gmt default parameters
gmtset [ -GfPdefaultsfile ] PARAMETER1 [=] value1 PARAMETER2 [=] value2 PARAMETER3 [=] value3 ...
gmtset will adjust individual GMT defaults settings in the current directory's .gmtdefaults' file. If no such file exists one will be created. The main purpose of gmtset is to temporarily change certain parameters inside a shellscript, e.g., set the dots-per-inch to 600, run the script, and reset to 300 dpi.
PARAMETER value
Provide one or several pairs of parameter/value combinations that you want to modify. For a complete listing of available parameters and their meaning, see the gmtdefaults man page.
-G
Name of specific .gmtdefaults file to modify [Default looks first in current directory, then in your home directory, and finally in the system defaults].
To change the dpi to 600, set anotation font to Helvetica, and select grid-crosses of size 0.1 inch, and set anotation offset to 0.2 cm, try
gmtset DOTS_PR_INCH 600 ANOT_FONT Helvetica GRID_CROSS_SIZE 0.1i ANOT_OFFSET 0.2c
gmt(1gmt), gmtdefaults(1gmt)