They can happily exist next to each other. In fact, they complement each other nicely in some areas, for example creating a ZFS pool on GEOM ELI providers which allows for hardware-accelerated full disk crypto for ZFS without having to wait for some ZFS version that may or may not implement the same thing.
GEOM NOP is handy for block device stability testing, and GEOM mirror and stripe + FFS are a good option for RAID on lower end machines where ZFS does not show its true potential (I'd say 64-bit arch with a 4 GB RAM minimum for that, and then you cannot cram many drives in without running out of cache space).
ZFS operates in a specialized market (enthusiast/professional/enterprise file storage systems), where GEOM is more an all-round storage stack that can be used as a foundation to build these file systems on.