Are the external drive encrypted?
Try rebooting the laptop since Finder & Disk Utility can get into a persistent weird and confused state at times if something unexpected happens and only a reboot will resolve the problem (at least until the event is triggered again).
Try a PRAM Reset and it never hurts to do an SMC Reset (follow the instructions for the T2 Mac).
Try accessing these drives while booted into Safe Mode.
Check to see which chipset your various G-Drives use by checking in the System Profiler (under Storage, or USB, or Thunderbolt -- not sure which is best). You can access the System Profiler by Option-clicking the Apple menu and selecting the first item (the Option key toggles the first item on the menu and perhaps other menu items). Maybe Monterey has a problem with certain USB chipsets used in external drives. I know Big Sur and/or M1 Macs had these types of compatibility issues last year as there were lots of posts here on these forums, but I've never really seen any in-depth article or analysis of the issue except that the JMicron USB chipset was fingered to have problems -- perhaps some other ones as well.
Use the Terminal app to try to manually mount the external drive to see if it may provide an actual error message. You will need to correct drive identifier for the volume on the external drive. You can get this by using Disk Utility. Using your screenshot as an example the drive identifier is "disk2s2", you must verify the drive identifier each time you mount the drive because the identifier will likely change. Here is the sample command where I use a generic drive identifier of "diskX" so make sure to replace "diskX" in my example with the correct drive identifier such as "disk2s2":
diskutil mount readOnly diskX
If this fails, then it may be interesting to see the output of the following command when run from Monterey and from the other MBPro where the drives are working. It is best to disconnect all other external drives so that only the information about the G-Drive is displayed.
diskutil list external
Is the data on these G-Drives backed up somewhere else? You should always have frequent & regular backups of the computer as well as any external media which contains important & unique data.