Let me start by saying that one of the downsides/upsides of PCs vs. Macs is that there are so, so, so many to choose from. I'm not going to suggest any specific one because there are too many to count! Macs only give you a few options. I am, however, going to give you some pointers as you shop around.
A decent gaming laptop should already have a GPU in it, so getting an external one wouldn't be needed. Is there a reason you're looking into getting a laptop? You get more processing power for your money with a desktop or a workstation.
That being said, if you match the specs of the iMac (but maybe bump the RAM up to 64 and have two SSDs - one for the OS and one for AE's cache and a third HDD for your footage [if necessary]), you should be just as happy.
Main tips:
- CPU
AE is using multiple cores more and more with each new version, but a lot of it is still rendering on one core, so getting the fastest core speed possible is a lot more important than having a ridiculous number of cores. - RAM
You want lots of RAM. - GPU
The GPU isn't used by a whole lot in AE, but each release sees more and more effects being accelerated by having one. For example, Fractal Noise (an effect I use a lot on every project) is accelerated by the GPU (and I love it). Also, if you do VR work, you must have a dedicated (and somewhat decent) GPU for AE's VR effects. Also, also, some third-party plugins (like Element 3d, Red Giant's Universe and Particular, Stardust, Zaxwerks, etc.) render entirely on the GPU or are accelerated by having one. Depending on what you plan to do, the GPU may not matter much or it may be essential.
Third also: If you plan to get into actual 3d work with Cinema 4D, you may end up using a third-party GPU renderer like Cycles 4D, Redshift, or Octane. In that case, you need to get as good of an NVIDIA card as you can (in the GeForce family, not Quadros. Quadros are pointless for our industry). Or, possibly two cards. Or maybe four. Look, I don't know what kind of work you do 
- Hard Drive
You want a fast SSD for AE's cache and you want it to be separate from your OS and files. (But, for overall machine performance, you want an SSD for your OS too, so you want at least two SSDs.)
If you want some more scientific(ish) data, Puget Systems (a great company from all I've heard) have put a lot of work into bench-testing various hardware to see what performs the best in After Effects and other software. Their white papers are not gospel, but they are educational reading.
Brand-wise, If you must get a laptop and you want a GPU, a lot of my friends who do 3d stuff and use GPU renderers are going with Razers (or is it Razors?). No matter how it's spelled, coming from a Mac, you'll probably want to get a sticker to cover up that ugly logo on the lid, but the hardware is good.
If mobility isn't essential and you get a desktop instead, Velocity Micro and Puget Systems are two brands that focus on content creators. If you like to get a little more custom without actually having to build one yourself, Digital Storm lets you custom-order the absolute heck out of a system.
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