GPL issue in an open call project

This issue summarise a conversation we had with @steffanlloyd and @robosurvey on a license issue:

Hey David, Philippe,

Steffan Lloyd here, I'm with NIBIO and I'm in charge of supervising the "Below-canopy mapping" open call. Ioannis (CC'ed) is the main developer on the open call.

We had a meeting today and Ioannis raised the concern that much of their work relies on the repositories listed below. The issue is that FAST_LIO uses the GPL-2.0 license, whereas the FUEL uses GPL-3.0. I understand you prefer copyright licenses, which these GPL uses will prevent. Will this be a problem? If so, what are our options? It would not be feasible to reproduce or redevelop this work from scratch.

https://github.com/hku-mars/FAST_LIO https://github.com/gisbi-kim/FAST_LIO_SLAM https://github.com/HKUST-Aerial-Robotics/FUEL

Let me know your thoughts. Best, Steffan