As I am sure you are all aware there is more than one Gostomysl recorded in the deep Russian history.
On Geni at the moment we have an MP,Gostomysl, Leader of the Ilmen Slavs which appears to be a meddling of the two men.
Gostomysl, son of Cedrag, can be read about here: https://ru.wikipedia.org/wiki/%D0%93%D0%BE%D1%81%D1%82%D0%BE%D0%BC%...
The other Gostomysl, father of Umila, can be read about here:
https://ru.wikipedia.org/wiki/%D0%93%D0%BE%D1%81%D1%82%D0%BE%D0%BC%...
or
https://en.wikipedia.org/wiki/Gostomysl
There is another relevant Geni profile: Gostomysl, Grand Prince of the Obodrites who is the son of Cedrag, Prince of the Obotrites which is a duplicate of the MP Cedrag, Prince of the Obotrites.