the grossglockner is due south of Zell am see, and is a full day out in itself. There are lots of little stops along it, musuems, craft type places, scenic vistas, cafes, play areas for the kids, etc. One point we stopped and had a snowball fight! It was boiling hot and we were in shorts and tops, and throwing snowballs, it was so much fun!
Then at the top you can see the grossglockner mountain (highest in Austria), the Pasterze glacier (at 8.4 kms the widest in the eastern Alps) and there is a big visitor centre. You can walk down to the glacier, or hike through several long tunnels to a big alpine area. We were lucky enough to see loads of marmots up there. It is a toll road too, ?32 for a day, but def well worth it if you love mountains.
In theory you could drive to the end of it, double back a bit then turn off to Italy but it would be too much to do if you planning to drive all the way down to Garda. gmaps says 6 hours, but the traffic through the grossglockner is leisurely, and it could be up to 10 hours without stopping anywhere.
There is a part in the middle, Hochtor, which is the highest point of the overall road. There is a huge car park - it's forbidden to larger vehicles, then a queue of traffic winding their way up it. It is very scary, and lots of clutches burning from cars trying to climb it at slow speed! Only try that bit if you are confident drivers! The view from the top is fantastic though, there is parking to stop for a bit up there.
The website is here