There are external GPUs for laptops, they usually connect through an ExpressCard slot. They aren't an ideal solution though, as the ExpressCard bandwidth is similar to PCI Express x1. This means that any gaming GPU would be severely bottlenecked by the ExpressCard slot, resulting in very poor performance.
There is talk of enclosures for graphics cards to connect externally coming out that can be connected with USB 3.0. Such a device is not on the market yet, and wouldn't do you any good if your laptop did not support USB 3.0 USB 3.0 would be a bit better than Express Card, but would probably still cause a severe bottleneck.
In short, if you want to game on a laptop, you're generally stuck with whatever graphics it comes with. Laptops with decent gaming graphics are always quite expensive, $1000 at a minimum. There are a few gaming laptops that do have upgradeable graphics, but they cost at least $1500, and laptop GPUs are not available to retailers, only to system manufacturers, so finding an upgrade usually involves digging around ebay for a GPU somebody ripped out of their laptop.
