You will only be able to extend a partition if the area you want to merge it with is directly next to the one you want to extend. In this case only E:\ or I:\ can be extended. Nothing you can do to force it to merge with the C:\ drive. You will need to move the unpartitioned space to in front of the E:\ drive so you can merge it with the C:\ drive. Gparted can do this for you. But it will take time!!!! You are talking about moving over 500GB of used space for 50GB of unpartitioned space. So it will take hours to move all of that data in order to accomplish what you want.
Thank you very much! So it is possible but will take a lot of time if I understand you right. Do you think that it will be faster and possible to copy E:\ to the external drive, delete partition E:\ and extend C:\ then make E:\ again and move its material back?
If you have space to move the data on E to an external drive, then copy it back again after expanding C, that would be a better option.
It will still take just as long, but you will end up with the new copy of your data partition being nicely defragmented.
Don't try it if E is not just a data partition though.
You can't move an OS piecemeal in that way. It won't boot again.