Mithalogica, that's a humongous amount of assets!
As Rich said, your best bet is to create sub-folder hierarchies for each asset category in your hard-disk, and then add their parent folders to Gimp's: Edit->Preferences->Folders .They will then appear as tags, again as Rich shown.
I know it will take time, and the more assets you add, the more stress you put to the program (so try to add only those you really need).
As for multiple machines, I don't think Gimp has a cloud, but you can always copy your main assets folder to a USB stick (that is besides backing it up to your dropbox account and re-D/L it from different machines).
As Rich said, your best bet is to create sub-folder hierarchies for each asset category in your hard-disk, and then add their parent folders to Gimp's: Edit->Preferences->Folders .They will then appear as tags, again as Rich shown.
I know it will take time, and the more assets you add, the more stress you put to the program (so try to add only those you really need).
As for multiple machines, I don't think Gimp has a cloud, but you can always copy your main assets folder to a USB stick (that is besides backing it up to your dropbox account and re-D/L it from different machines).
Gimp & Stuff: https://www.deviantart.com/migf1