Is GPL-2.0-only compatible with GPL-3.0-only?
No.
Different copyleft versions with no upgrade clause cannot be merged. Resolve by finding a 'GPL-2.0-or-later' variant of the dependency.
General guidance, not legal advice.
Obligations when redistributing as open source
No obligations checklist — this combination cannot be distributed as a single work.
Other distribution models
The verdict is the same under every distribution model; the obligations differ. Check this pair in the tool to switch models.