7.9 and 8.1 gateways can connect to each other, but there are compatibility limits. Projects and project resources cannot be transferred between 7.9 and 8.1 (in either direction). Tag values can be read across the gateways, but the tags themselves cannot be remotely edited. So it really comes down to what your gateway network is being used for. As always, we strongly recommend that you set up development environments to test 7.9 to 8.1 compatibility before attempting any production upgrades.