I am in the process of transferring my Reunion for Mac family tree to Gramps. Now I have a question. Where do you store data such as: height, eye color, weight and other physical characteristics? Do you do that in notes?
You should use attributes which correspond to Gedcom facts.
Are there specific GEDCOM codes for this?
There are some pre-defined Gedcom individual attributes. These should be mapped to Gramps attributes. If you define a custom attribute in Gramps, then it will be mapped to a Gedcom Fact.