In one of my machines, debug was enabled in /etc/gssproxy/gssproxy.conf
(so it tells me that, contrary to the message from the daemon, this file
is indeed read, see #950514). It was probably done before the strech->buster
update.
  I saw that, when debug is enabled, gssproxy segfault at start.
Removing the debug option makes gssproxy working.

# cat /etc/gssproxy/gssproxy.conf
[gssproxy]
debug=true
debug_level=2
# tail /var/log/syslog
[...]
Feb  2 23:06:06 ge91098 systemd[1]: Starting LSB: Controls operation of the gssproxy daemon...
Feb  2 23:06:06 ge91098 gssproxy[21077]: [2020/02/02 22:06:06]: Debug Enabled (level: 2)
Feb  2 23:06:06 ge91098 gssproxy[21077]: [2020/02/02 22:06:06]: Client [2020/02/02 22:06:06]: (/usr/sbin/gssproxy) [2020/02/02 22:06:06]:  connected (fd = 10)[2020/02/02 22:06:06]:  (pid = 21085) (uid = 0) (gid = 0)
Feb  2 23:06:06 ge91098 kernel: [258173.020117] gssproxy[21085]: segfault at 0 ip 00007fa622cf8e5a sp 00007fffe3e72e30 error 4 in libselinux.so.1[7fa622cec000+25000]
Feb  2 23:06:06 ge91098 kernel: [258173.020127] Code: c7 45 00 16 00 00 00 e9 df fe ff ff 0f 1f 40 00 bf 01 00 00 00 45 31 f6 e9 4c ff ff ff 0f 1f 00 41 55 41 54 55 53 48 83 ec 08 <4c> 8b 27 49 8b 3c 24 48 85 ff 74 05 e8 95 91 ff ff 49 8d 5c 24 08
Feb  2 23:06:06 ge91098 systemd[1]: gssproxy.service: Succeeded.
# systemctl status gssproxy
● gssproxy.service - LSB: Controls operation of the gssproxy daemon
   Loaded: loaded (/etc/init.d/gssproxy; generated)
  Drop-In: /etc/systemd/system/gssproxy.service.d
           └─restart.conf
   Active: activating (auto-restart) since Sun 2020-02-02 23:53:14 CET; 991ms ago
     Docs: man:systemd-sysv-generator(8)
  Process: 21645 ExecStart=/etc/init.d/gssproxy start (code=exited, status=0/SUCCESS)
  Process: 21659 ExecStop=/etc/init.d/gssproxy stop (code=exited, status=0/SUCCESS)

Note: the drop-In restart.conf file is here to make the service automatically
restart in case of failure: the service is required on my machines.

  Regards,
   Vincent

We believe that the bug you reported is fixed in the latest version of
gssproxy, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 950518@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Robbie Harwood (frozencemetery) <rharwood@club.cc.cmu.edu> (supplier of updated gssproxy package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Wed, 05 Feb 2020 19:17:32 -0500
Source: gssproxy
Architecture: source
Version: 0.8.2-1
Distribution: unstable
Urgency: medium
Maintainer: Robbie Harwood (frozencemetery) <rharwood@club.cc.cmu.edu>
Changed-By: Robbie Harwood (frozencemetery) <rharwood@club.cc.cmu.edu>
Closes: 856965 875468 950518
Changes:
 gssproxy (0.8.2-1) unstable; urgency=medium
 .
   * New upstream version 0.8.2 (Closes: #950518)
   * Bump standards version and lintian clean
   * Switch to upstream's systemd unit (Closes: #875468)
   * Stop providing NFS server config snippet (Closes: #856965)
Checksums-Sha1:
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 d398b27cc9042cfea79f6880f00bd9c3d54436bb 168202 gssproxy_0.8.2.orig.tar.gz
 71b311df7ed1f6e5b4bfa6348742c129c99f99ea 3952 gssproxy_0.8.2-1.debian.tar.xz
 2f647e4af47fca0a64d12f0ea9a07f25a65a74fc 7659 gssproxy_0.8.2-1_source.buildinfo
Checksums-Sha256:
 9f61dff5e9310101001db3eb880de8c05ce2a5739e1fca0cefcc7e60ae2e1bab 2350 gssproxy_0.8.2-1.dsc
 95691ed8f994d911925f370d924bff2c8e9946c8478b0de069a751c91bc801cd 168202 gssproxy_0.8.2.orig.tar.gz
 6b6591ac01401d90604ec1373d19c2b259b3ff19eb65e813e2abf3e03582967a 3952 gssproxy_0.8.2-1.debian.tar.xz
 b8628eba881c5b0c51c5fbf2cc4fdd816937f3d3cf40477d32e1741b8e7bf3bf 7659 gssproxy_0.8.2-1_source.buildinfo
Files:
 d9e49caa89ff5bef9d6698d05ead35ef 2350 net optional gssproxy_0.8.2-1.dsc
 0482543d7d0901af0fbfa5807956d62c 168202 net optional gssproxy_0.8.2.orig.tar.gz
 127d6ab412ab6c81e8fb3b8ceb4b2f61 3952 net optional gssproxy_0.8.2-1.debian.tar.xz
 daeb19106b4c6cdf1d438a9ba9b0ed82 7659 net optional gssproxy_0.8.2-1_source.buildinfo
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  Hi,

  I backported gssproxy/0.8.2-1 (gssproxy/0.8.2-2 is not fully commited in
the git repo) and saw that this bug is still present:

Feb  4 11:01:37 ge132116vm-1 systemd[1]: Starting GSSAPI Proxy Daemon...
Feb  4 11:01:37 ge132116vm-1 gssproxy[1455]: [2021/02/04 10:01:37]: Debug Enabled (level: 1)
Feb  4 11:01:37 ge132116vm-1 systemd[1]: Started GSSAPI Proxy Daemon.
Feb  4 11:01:37 ge132116vm-1 gssproxy[1455]: [2021/02/04 10:01:37]: Client [2021/02/04 10:01:37]: (/usr/sbin/gssproxy) [2021/02/04 10:01:37]:  connected (fd = 10)[2021/02/04 10:01:37]:  (pid = 1456) (uid = 0) (gid = 0)
Feb  4 11:01:37 ge132116vm-1 systemd[1]: gssproxy.service: Main process exited, code=killed, status=11/SEGV
Feb  4 11:01:37 ge132116vm-1 kernel: [471673.673095] gssproxy[1456]: segfault at 0 ip 00007fc521abfe5a sp 00007ffe0a035a50 error 4 in libselinux.so.1[7fc521ab3000+25000]
Feb  4 11:01:37 ge132116vm-1 kernel: [471673.673103] Code: c7 45 00 16 00 00 00 e9 df fe ff ff 0f 1f 40 00 bf 01 00 00 00 45 31 f6 e9 4c ff ff ff 0f 1f 00 41 55 41 54 55 53 48 83 ec 08 <4c> 8b 27 49 8b 3c 24 48 85 ff 74 05 e8 95 91 ff ff 49 8d 5c 24 08
Feb  4 11:01:37 ge132116vm-1 systemd[1]: gssproxy.service: Failed with result 'signal'.

# apt-cache policy gssproxy
gssproxy:
  Installé : 0.8.2-1~bpo10+1


Removing the debug line in the config allows gssproxy to start.

  Regards,
    Vincent