Highlight your grandson and do an Add Spouse to add his girlfriend as if she was his wife. Just don't add a Marriage fact since there isn't a marriage.
Highlight your grandson again and do an Add Child to add your great grandchild. The child's surname will default to your grandson's surname, so type over the default surname with the surname of the mother.
You are really done at this point, but if you wish you could define a fact called Partner or some such and then give that fact to your grandson and his girlfriend. Be sure the Partner fact or whatever you choose to call it is a Family fact and not an Individual fact.
I'm sure that doing an Add Spouse will feel a little off-putting because outside of RM the term "spouse" typically implies a marriage. But that's the way you do it in RM. Be aware that the term Spouse may appear on your computer screen in RM, but it will not appear anywhere in any RM reports that you might run. Reports are going to include a Marriage fact if one exists and are not going to include a Marriage fact if one does not exist. That's really the only difference between Spouses and Partners in reports, unless you add a Partner fact to further emphasize the difference.