Ok, I'll do that now.
In the mean time I built a core-image-x11 for imx6qdlsabresd with following changes in local.conf:
> 
> 
> DISTRO ?= "fslc-x11"
> ACCEPT_FSL_EULA = "1"
> IMX_DEFAULT_BSP = "nxp"
> CORE_IMAGE_EXTRA_INSTALL += " \
> packagegroup-fsl-gstreamer1.0-full \
> "
> 

With none of my own layers enabled, and the problem persists.

> 
> root@imx6qdlsabresd:~# uname -an
> Linux imx6qdlsabresd 5.4.70+g2d1241e38347 #1 SMP PREEMPT Wed Oct 7
> 17:10:48 UTC 2020 armv7l GNU/Linux
> root@imx6qdlsabresd:~# DISPLAY=:0 gst-launch-1.0 videotestsrc !
> imxeglvivsink
> Setting pipeline to PAUSED ...
> Xlib:  extension "XFree86-DRI" missing on display ":0".
> Pipeline is PREROLLING ...
> Xlib:  extension "XFree86-DRI" missing on display ":0".
> Xlib:  extension "XFree86-DRI" missing on display ":0".
> Pipeline is PREROLLED ...
> Setting pipeline to PLAYING ...
> New clock: GstSystemClock
> Caught SIGSEGV
> exec gdb failed: No such file or directory
> Spinning.  Please run 'gdb gst-launch-1.0 474' to continue debugging,
> Ctrl-C to quit, or Ctrl-\ to dump core.
> 
>