Is lenovo g400 laptop good for gaming?

Depends on the GPU. Assuming it's the Radeon 8750 w/ GDDR5 (as Lenovo's site states "up to" an 8750), it's good. But the G400 can come in a variety of configurations, and the 8750 itself can also come in different configurations varying in clock speed and memory type. If it comes with only DDR3 instead of GDDR5, then the GPU (and laptop as a whole) goes from "good" to "good enough", managing to play most games at lower resolutions with lower settings. In general, the 8750 should be able to play demanding games at 720p resolution, medium settings, above 30FPS. Again, this is assuming the specific G400 you are inquiring about even comes with a 8750. It may come with another GPU entirely.

The short answer: it should be good enough.

First of all laptops are not good enough to handle latest games and you can stick with it for less time as it will quickly become outdated. The core i5 3rd gen (as per site) and 6gb is ok for gaming. 2gb graphics is not enough spec. GTX 660 ti is 2gb gpu and a gtx 610 also has 2gb memory but performance wise 660 ti is way ahead than 610. From lenovo site, it has HD8750M gpu. Have a look at the review and game play fps below,
http://www.notebookcheck.net/...147.0.html
For gaming only the gpu plays most important role not cpu so if there are any other option where you are getting a better gpu than 8750m and relatively inferior cpu than i5 3rd gen then it will be a better performer than current one. This laptop is ok for entry level gaming only. But with in one and half year your spec will be outdated for gaming. So if your requirement is less then it is ok for you. Otherwise you have to spend much more for main stream gaming on laptop. Again laptop is not a good choice for gaming if you don't have enough to spend.
I have purchased hp pavilion with i7 3rd gen+hd6770m gddr5+8gb ram. After 1 year I found difficulties with latest games (less than 35fps). So I have sold it in half price and purchased a custom rig (i5-3450, 660 Ti jetstream, 8gb ram, 128gb ssd) and after 1 year my pc is very much comfortable with latest games (cod ghosts, 50+ fps in near to max setting). So decide wisely.

Depends on the games your are wanting to play on the computer however those specs are fairly good for a laptop so you should be fine with most new games.