Not sure, but in my experience GDBM is a bit on the slow side. MDBM uses mmap(), so for that reason alone it should be faster.
MDBM was designed to be fast with special care taken on the lookup path. The goal was to do lookups with as few cache misses as possible. You can get to any key with at most two page faults.
They might appear similar, but that's just because they share the same DBM interface heritage.