Since version 7.7, the system.util.retarget command does not work between Gateways with different Ignition versions, we are forced to resort to a very little smart trick and the jump between gateways is not seamless for the operator anymore. Please fix it.
What specific version of 7.7 are you using? There was an issue with retargeting between major release versions, this issue was resolved in Ignition 7.7.5. If you are not on this version I would suggest updating.
The versions installed in the gateways are 7.7.6 and 7.6.6, when retargeting from V7.7 to V7.6 it works properly, however when doing the opossite, the message in the attached image is shown.