Dear Stephan,
the section from Omegna to Gravellona is much easier, about class 3, with nice rapids, but not so nice as enviroment, since in that portion the river crosses the industrial areas of both Omegna and Gravellona.
If you decide to run it, the put in is in Omegna, where the Nigoglia tributary (coming from Lake Orta) is flowing in the Strona. The take out is in Gravellona, after the bridge, on the right bank, just before the artificials are beginning.
Be careful! More or less at half way you meet an artificial partially collapsed with a syphon. It MUST be portaged on the left bank.
Ciao,
Stefano