iCloud has sucked up all my laptop's files - can you transfer files directly from iCloud to Google Drive?
I have to agree with the comments:
1.) Get an external drive
2.) Offload or delete what you can on your mac.
3.) download some stuff you need, copy it to a local harddrive (out of iCloud), delete it in iCloud, then upload to Google Drive.
4.) Repeat 3.) until done.
iCloud can take a long time to sync up completely, so be patient.
Here is what Apple has to say: https://support.apple.com/en-us/HT206996