Since 2023 Nov, GCKey page says submission is no longer processed.
This was how I finally got through IRCC work/activity history section.
1. enter education as necessary.
2. First, just say unemployed for 10 years. (from 2024 Jan, ongoing) Try "Save and Proceed"
3. If it passes, then click "Back", you will come back, and previous entries should all still be there. Then modify that 10-year unemployment entry to your latest employment (say 2022 Jan to 2024 Jan), then add another unemployment (from 2014 Jan to 2022 Jan).
4. Then try "Save and Proceed".
5. If it works, just repeat 3 and 4 for each entry. In this pattern, you would be adding your exp from the latest/closest to the earliest.
6. There should be no gap, I had to add "unemployment" for my undergraduate time. But job time and education time can overlap, no problem for me.
7. I do not know what happens if two jobs overlap.
8. If you have to omit some experiences/entries to pass this section, include them in the final "additional notes", "optional documents" at the later "Document submission" stage, just before payment.
Hope this would work for someone.