Problem
After upgrading to Ubuntu 20.04 form 18.04, I wasn’t able to start GIMP 2.10. The message was in the same format as that in a related Ask Ubuntu question, except that the version numbers were greater.
I attempted installing the package gegl, which was irrelavant to the problem.
Analysis
I found the answers mentioning the package libgegl helpful.
$ dpkg -l | grep gegl
ii gegl 0.4.22-3
amd64 Generic Graphics Library Test Program
ii libgegl-0.4-0:amd64 1:0.4.18+om-0ubu18.04.18~ppa
amd64 Generic Graphics Library
ii libgegl-common 1:0.4.18+om-0ubu18.04.18~ppa
all Generic Graphics Library - common files
Despite deletion of ppa:otto-kesselgulasch/gimp by ppa-purge, that leaves a
trace in /etc/apt/sources.list.d.
Solution
A related question on Ubuntu Francophone Forum contained the right command to solve the problem.
$ sudo apt-get install --reinstall libgegl-0.4-0=0.4.22-3 \
> libgegl-common=0.4.22-3
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be DOWNGRADED:
libgegl-0.4-0 libgegl-common
0 upgraded, 0 newly installed, 2 downgraded, 0 to remove and 0 not upgraded.
Need to get 1,519 kB of archives.
After this operation, 194 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://archive.ubuntu.com/ubuntu focal/universe amd64 libgegl-0.4-0 amd64
0.4.22-3 [919 kB]
Get:2 http://archive.ubuntu.com/ubuntu focal/universe amd64 libgegl-common all 0
.4.22-3 [599 kB]
Fetched 1,519 kB in 3s (558 kB/s)
dpkg: warning: downgrading libgegl-0.4-0:amd64 from 1:0.4.18+om-0ubu18.04.18~ppa
to 0.4.22-3
(Reading database ... 228752 files and directories currently installed.)
Preparing to unpack .../libgegl-0.4-0_0.4.22-3_amd64.deb ...
Unpacking libgegl-0.4-0:amd64 (0.4.22-3) over (1:0.4.18+om-0ubu18.04.18~ppa) ...
dpkg: warning: downgrading libgegl-common from 1:0.4.18+om-0ubu18.04.18~ppa to 0
.4.22-3
Preparing to unpack .../libgegl-common_0.4.22-3_all.deb ...
Unpacking libgegl-common (0.4.22-3) over (1:0.4.18+om-0ubu18.04.18~ppa) ...
Setting up libgegl-common (0.4.22-3) ...
Setting up libgegl-0.4-0:amd64 (0.4.22-3) ...
Processing triggers for libc-bin (2.31-0ubuntu9.1) ...