AdultSwim, the publishers of GBoD always produce fun and easy to play games and this is no exception. The game itself is free to download and play, but as many games these days it is based on the Freemium model. I'm ok with that in this case because the premium items and extra themes are well priced.
Here are some screenshots I grabbed of the start screen and the beginning of your boulder roll.
You can also upgrade your boulder and chance items. The chance items are access via a pokies type spinner at the start of every game. You get a free roll every 10 minutes and this gives you little perks in the game like reducing the number of spikes or adding Yetis.
There's also a gallery of victims, basically all the stuff you have unlocked so far.
So the idea with the game is to roll the boulder and squash everything in sight while not being spiked or exploded by bombs or falling off into the abyss. Pretty simple! Your boulder has a power meter too, which when full will turn the boulder into an 'Invinciboulder' (for a short period) which can't be destroyed and can squash anything, even the spikes and giant buildings.
If you get spiked or exploded you die, but you can continue by using gems. The first time you die it costs 1 gem, the second time it costs 2 gems, after that you can't even use gems to continue. If you fall into the abyss below you can't continue either.
There are extra 'themes' available to download that change the look of the game and essentially are treated as new levels. Some of these cost gems and some cost actual money to purchase.
Now go and get GBoD!