So i just installed Ubuntu Gnome 15.04, and me has issues. First, I installed the latest drivers on nvidia's website for my 760. When i ran the .run thing, after the bar filled up, GEDIT came up with an error message and started to eart up 6+ GB of my 8gb RAM. Halp. Second issue, i did sudo apt-get install steam, and after it downloaded, it launched and went through the whole Updating Steam thing. After it finished, i ran steam and it didnt load, halp plz.
But overall i'm in love with linux and i get why you guys bitch and "WINE" when theres no support (duh dum tssssss).
Weird that steam isn't working for you. I am running Ubuntu Gnome 15.04 myself and no issues here.
I would try purging steam and then updating your system if you have not already. Then try to reinstall steam.
I always like to remove and then purge. Something dependencies are left behind then you can do an autoremove.
As for your Nvidia card I wouldn't use the driver's from their website. Never works. Watch this YouTube video I also have a GTX 760 and works fine. I'm running the nvidia-352 drivers and is stable.
If nothing works you can always wipe and reinstall Ubuntu and try again. Hope that helps.
My Laptop runs on a gtx950m so i can't say i share the same card. But what I will say is that i find using the Nvidia PPA a lot easier whenever i use Ubuntu.
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
sudo apt-get install nvidia-355
This isn't a fix, just might be an idea for next time you install your gpu drivers. PPA's = easier.