neither copy or sync will ever delete anything from the source, so nothing in your GPhotos: is at risk in any way. Sync is appropriate for this job if you want the destination to always be the same as the source (including removing any files on the destination that you choose to remove on the source).
Just don't use move obviously.
Besides, if you are ever in any doubt - try using
--dry-run
this will just simulate the operation and give you the output of what would happen, but transfers actually being performed. You can verify that you didn't make a blunder in the way you wrote the command before actually running it.
You probably also have to use
-v (verbose output)
if you want to see extended details about what happens.
-P or --progress is also just generally useful as it will display real-time stats of the transfer, like speed, files done/remaining, what files are transferring right now ect.