No you do not get new scenarios or campaigns, Gustav Line is the first official expansion to Fortress Italy which includes new scenarios, campaigns and TOE's. The version 3.0 upgrade brings Fortress Italy up to Red Thunder engine standards which you can read about on the product page or the patch notes itself.
Edit: Yes it is fully compatible with existing scenarios, campaigns and saved games. However in regards to saved games, strange things do sometimes happen, but they will be playable.