If all your machines are 64 bit installs, then this issue is not something you will likely come across.
However if you have some 32 bit and some 64 bit and try and take mydb.gdbm across from 32 bit to 64 bit machine, or vice versa, then look out.
gdbm is wordsize specific. That means that your sample 10 row database created on a 32 bit machine, will look different than the same 10 row database created on a 64 bit machine.
gdbm fatal: lseek error
Not the most helpful message, but should you encounter this message then you have probably hit the issue I described above.
Solution: Create your data on the appropriate architecture machine.
Round robin databases (.rrd) have a similar issue and this restriction is long understood.
GDBM files are not portable between different architectures.
If a database being wordsize specific, is a serious limitation for you, then gdbm and rrd are probably not the technology you should invest your time in. Perhaps sqlite or mongodb are alternatives you might consider.
Notes and Further Reading:
gdbm was originally created as part of the GNU project.
The GNU project acted as an incubator and first implementation for this project, which has now gone on to maturity / stability.