Exporting pre-defined event types will give the following:
BURI
TYPE <description>
For custom events you will get:
EVEN <description>
TYPE <event_type>
The mapping between Gramps event types and the pre-defined Gedcom tags can be found in the libgedcom.py file. I think that all the Gramps types map to a tag except for the new Stillbirth type that we are adding in the next release. [see Stillbirth proposal]
The import should handle any Gedcom exported by Gramps and also some extra cases for other software. There is addition code to handle relationship types and nobility titles.