Hi John --
I am unable to recreate the issue using a clean Buster environment. However, there are some deltas in your dpkg output vs. mine:
root@6a9a828130f5:/usr/src/freeswitch# dpkg --list | grep gdbm
ii libgdbm-compat-dev 1.18.1-4 amd64 GNU dbm database routines (legacy support development files)
ii libgdbm-compat4:amd64 1.18.1-4 amd64 GNU dbm database routines (legacy support runtime version)
ii libgdbm-dev:amd64 1.18.1-4 amd64 GNU dbm database routines (development files)
ii libgdbm6:amd64 1.18.1-4 amd64 GNU dbm database routines (runtime version)
root@6a9a828130f5:/usr/src/freeswitch# ls -al /usr/lib/x86_64-linux-gnu/libgdbm*
-rw-r--r-- 1 root root 113618 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm.a
lrwxrwxrwx 1 root root 16 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm.so -> libgdbm.so.6.0.0
lrwxrwxrwx 1 root root 16 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm.so.6 -> libgdbm.so.6.0.0
-rw-r--r-- 1 root root 63512 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm.so.6.0.0
lrwxrwxrwx 1 root root 23 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm_compat.so -> libgdbm_compat.so.4.0.0
lrwxrwxrwx 1 root root 23 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm_compat.so.4 -> libgdbm_compat.so.4.0.0
-rw-r--r-- 1 root root 14104 Mar 12 2019 /usr/lib/x86_64-linux-gnu/libgdbm_compat.so.4.0.0