I think exact specifications will be hard to find as it all depends on what type of simulation you're running, the version of Gazebo and multiple hardware components. But I think any new laptop (i5 or i7, certainly if you're looking at models with the GTX 860M) should be no problem. Ubuntu runs very very efficiently compared to my normal Windows 7 installation on my Lenovo ultrabook.

Found these threads as well, they have some useful info/guidelines. Maybe check them out to get a general idea: http://answers.gazebosim.org/question/7982/tested-graphic-card-list-for-gazebo/ http://answers.gazebosim.org/question/5559/dual-processors-with-gazebo/ http://answers.gazebosim.org/question/25/run-gazebo-in-virtualbox/

And my experience on Virtual Machine, I tried ROS (no Gazebo yet) on a virtual machine before I installed Ubuntu dualboot and experienced even some lag in typing in the console so I expect any hardcore simulations might run but with very low realtime factor and choppy graphics. Save yourself the hassle (and lag) and just install Ubuntu dualboot.

I can concur that running it in a virtual machine is a bad idea. There are a lot of MacBook's on my team, and one of my colleagues was running Ubuntu inside VirtualBox on her MacBook, and it was very painfully slow/choppy for her. I installed Ubuntu directly (accidentally erased macOS--whoops!) and Gazebo ran very smoothly for me.