1) Why a laptop?
Do you plan to lug it around a lot (in this case get alight and small machine, which means sacrificing a lot of performance)?
Do you plan to work at different places around the house (In this case get a desktop replacement laptop, which means less sacrifice on performance, but a big and heavy machine you do not want to throw in your backpack)?
Do you plan to not move the machine from your desk and want to get a laptop just-in-case (in this case get a desktop, get full performance and save a lot of money)?
2) What are the performance requirements?
Do you want to build and test 2D casual games (in which case ANY machine will do most probably)?
Do you plan to do smaller, less complex 3D games (in which case you might want to get a discrete graphics card and not rely on an iGPU in the CPU)?
Do you want the biggest, fattest 3D Power you can get for Full HD (In which case you should start with a GTX 960 or R8 380 equivalent card)?
Do you want 4k at full details (in which case you need to plonk down 750$ just to game at 30 FPS today 
)?
3) Forget about "does not overheat"... the only laptop that doesn't overheat is one without a dedicated GPU. Now, this is grossly oversimplified, there are better GPUs in this regard than others (generally the smaller GPUs seem to fare better), but generally ALL dedicated Laptop GPUs seem to throttle to some degree. Throttling is only happening when the components start to overheat.
To give a little perspective: picking out one of the more attractive mobile GPUs (and actually one of the most powerful), the GTX970m, this card should actually beat a GTX960 (non-mobile) on paper. 256 more CUDA cores, more VRAM, more memory bandwidth. The lower clocks bring the card quite close to the GTX960 non-mobile again, but it should still be very hard for the non-mobile 2,3 TFLOPS card to flatout beat the 2.7 TFLOPS mobile card.
Yet if you look at benchmark websites, you see the GTX970m getting DESTROYED by the GTX960. With a 20% lead by the non-mobile card in some cases.
Now how can that be? There is only one explanation: throttling! The GTX970m must be constantly underclock because the cramped laptop cases just do not have the airflow. And given that these benchmark results get aggregated from thousands of benchmark results from a wide range of laptops, and yet the result is still so clear can only mean that most of the laptops equipped with GTX970m must be throttling to some degree.
Now this is just a single card from many, and a benchmark is hardly a realworld test. But it does indicate that laptop cases in general still are not up to the task of cooling all the heat the internal components can generate under full load.
My personal recommendation would be to really think long and hard if you NEED a laptop. You pay for the PC, the Keyboard, Mouse and Screen that are built into a laptop, AND a Premium for laptop components on top of that.
If you go for 3D gaming grade components, expect a 2000 bucks laptop to struggle to keep up with a self-built 750$ desktop. You might see a laptop with similar speed for 1300$, but then you will most probably have to compromise on screen and build quality).
That calculation changes of course if you also need a new screen, keyboard and mouse, but given that working with a touchpad is hardy ideal you will need to get a mouse anyway, and even though most gaming grade laptops have quite good chiclet keyboards, you might want to upgrade on that depending on your taste. That leaves the screen, but let me assure you, laptops with good screens are actually much more expensive than the ones with the crappy ones.
A Desktop will always give you more power for less money, leaving you with better upgrade possibilities down the road, and a more ergonomic work environment if you are happy to not be able to lug your rig around.
As for the OS selection... Mac OS if you want to develop for Mac/iOS exclusively (you need a test machine as close to your final target system as possible in the end), Windows for all other cases.
At the moment Windows is still THE Platform for PC gaming. You might also be able to make a Linux environment work, but prepare to invest more time into setting things up and finding tools that actually work on linux (or inside Wine, for that matter).
Why are all "professional" / "corporate" marketed laptops dual-core processors? And expensive as hell for crappier hardware? Something like Asus Zenbook which is marketed more as a "gaming" laptop is both cheaper and outperforms many "professional" laptops...
Generally everything with "business", "professional" or a similar monicker slapped on is a rip off. Big companies are able to pay big bucks for hard- and software, and if you need something for work, you are ready to pay more for it (hence the fantasy prices charged for the Quadro cards from Nvidia for example... same hardware as in the GTX range (save the ECC VRAM), different drivers = 5 times the Price... right, makes sense).
If I may guess though, the gaming laptops are to this day generally biggers and heavier than other laptops... companies start to sell ultrabooks as business hardware (some business people really start using iPads for work... now that is always a sight to behold (Masochists :) ))
