Using kde4, I launched gkrellm using

kstart --alldesktops --ontop gkrellm --geometry -0+0

which placed gkrellm at the top right of all work spaces.

Replacing kstart with kstart5:

kstart5 --alldesktops --ontop gkrellm --geometry -0+0

places gkrellm at the top left of all workspaces. It appears that kstart5 isn't sending command-line arguments. I tried

kstart5 --alldesktops --ontop 'gkrellm --geometry -0+0'

but that didn't even launch gkrellm.

Is there something I'm doing wrongly?

What I just tried and managed to get this to work without problems:
I installed gkrellm (and the numerous other packages needed to read sensors), then started it with Krunner. Once I get the krells stack I move it where I want it to be, then right click on the title bar to go to the configuration. Right click on the Task Manager entry to set it to all desktops and it just works.

You might have to try a few window specific settings to get exactly what you want, and might have to add it to the auto-start list in the System Settings, but for me it worked out of the box. Not sure using the same arguments you used in Plasma 4 are still valid for Plasma 5, but I am not using many GTK-based applications anyway.