When the Gramps GEDCOM importer plugin sees a custom tag, it adds the unrecognized content as a Note.
Is this content reversible?
That is to say, will the GEDCOM Export write the custom tag back to the same place in the structured data?
WikiTree is an example. They break out a Middle name in their internal data structure and do not include that name the standard GIVN Given Name value. Instead they write a _MIDN custom tag with that portion of the GIVN data.
This content is preserved as a Note for the Person. But it there enough context preserved to restore the data? Or even to do a meaningful post-processing of the ambiguous data.
I don’t see how since the Note is not attached as a secondary object to the same object. (The Note attached to the Person object, not to the more appropriate ‘preferred Name’ secondary object of that Person object.)
In the example of a WikiTree GEDCOM being imported, the “GEDCOM import” type Note is attached to the person with the content:
Records not imported into INDI (individual) Gramps ID I000003:
Line ignored as not understood Line 93: 2 _MIDN Franklin
Meanwhile, the original context in the “WikiTree_McCulloughWm1775.ged” GEDCOM source file starts at line 90 and runs through line 155:
0 @I3@ INDI
1 NAME William Franklin /McCullough/
2 GIVN William
2 _MIDN Franklin
2 SURN McCullough
2 NSFX Sr
1 SEX M
1 BIRT
2 DATE 24 May 1841
2 PLAC Westmoreland, Pennsylvania, United States
1 DEAT
2 DATE 4 Sep 1917
2 PLAC 136 Boyles ave, New Castle, Lawrence, Pennsylvania, United States
1 WWW https://www.WikiTree.com/wiki/McCullough-1311
1 FAMC @F2@
1 FAMS @F3@
1 NOTE == Biography ==
2 CONT
2 CONT '''William F. McCullough, Sr.'''
2 CONT
lines 108-147 are just concatenated line from the Biography. Continue with line 148
2 CONC />Lawrence county, Pennsylvania
2 CONT * {{FindAGrave|159862323}} for Wm. F. McCullough, Sr.
1 REFN 9567903
2 TYPE wikitree.user_id
1 REFN 10072967
2 TYPE wikitree.page_id
1 REFN 60
2 TYPE wikitree.privacy
0 @I4@ INDI
1 NAME Hollis Rushton /McCullough/
The Note about the misunderstood level ‘2’ Tag is attached to the level ‘0’ Individual but is actually secondary to the level 1 “NAME” for that individual. The Individual attachment is known in the imported data but the NAME attachment is lost.
The Gramps ID for the Individual is I0003 … but that correlation was contingent on being imported into an empty Tree. There is no direct reference to @I3@ of the GEDCOM.
Further, there are 3 REFN Attributes (9567903, 10072967, 60) added to Person for the final parts of the GEDCOM segment for this individual. But the wikitree.user_id, wikitree.page_id, and wikitree.privacy TYPE context is completely lost for those 3 REFNs.