Area-51m, two AW2518H, D6000 dock

You absolutely do not want to run displays through a D6000 dock on a gaming system. You should return that dock ASAP.  For an explanation as to why the D6000 is horrible for gaming, see my post in this thread, specifically the post marked as the answer: For an explanation of why the D6000 is terrible for gaming, see my post in this thread, specifically the one marked as the answer: https://www.dell.com/community/Alienware/Docking-Station-D6000-Very-Bad-Video-performance-for-gaming/td-p/6193369

If you want to use G-Sync (or any refresh rate above 60 Hz, according to the AW2518H's manual), you need to use native DisplayPort connections.  Your system has a Mini-DisplayPort output and a USB-C/TB3 output that supports video, the latter of which would provide a DisplayPort signal.  So you should attach one display with an mDP to DP cable and the other with a USB-C to DP cable.  However, for the display attached to the USB-C/TB3 port, whether you can use G-Sync and higher refresh rates will depend on whether that output is physically wired directly to the NVIDIA GPU (which is what you want) or wired to the Intel GPU with the NVIDIA GPU only providing content for that output via Optimus (bad).  If it's the former, then if you want to minimize cabling, you could alternatively use a USB-C dock or a Thunderbolt dock and you should still be able to get G-Sync.  I can't say for certain because G-Sync compatibility isn't mentioned either way on dock documentation.  However, also note that today’s USB-C docks max out at dual 1080p 60 Hz, which is fine for your current displays (if you don’t want anything past 60 Hz) but doesn’t allow any futureproofing.  Thunderbolt docks by comparison have access to 4x more display bandwidth, so they can do dual 4K 60 Hz or (in theory) dual 1080p 240 Hz. And a simple USB-C to DisplayPort cable like I mentioned earlier has enough bandwidth for a single 1080p 240 Hz display. I said “in theory” earlier because while the bandwidth is there to support the configurations I described, I don’t know if the Thunderbolt docks themselves might have limitations around 240 Hz and/or G-Sync since those aren't widely used features, so they're not often discussed.  And again, if the USB-C/TB3 output is wired to the Intel GPU, then any limitations around refresh rate from that GPU would apply, just like limitations around not being able to use G-Sync.

@abanceI edited my post above to make some fairly substantial revisions/expansions. Hopefully it helps.  For what it's worth, in terms of the question of which GPU your USB-C/TB3 output is wired to, if you open NVIDIA Control Panel, I believe if you go to the PhysX Configuration page it provides a diagram that shows which physical display outputs are wired to which GPU, so that might help.  Although it might only do that when there's an actual display attached via a given output, so you might have to actually connect a display via native USB-C/TB3 to see that.  Having a display connected to the D6000 wouldn't count even though the D6000 itself connects over USB-C because as I described in the post I linked above, the D6000 doesn't tap into any native GPU outputs at all.

If you find that the USB-C/TB3 port is wired to the Intel GPU, then the only way to run G-Sync on both of your displays would be to connect a DisplayPort MST hub to the Mini-DisplayPort output and connect both displays to that MST hub.  However, all DisplayPort MST hubs I've seen only support DisplayPort 1.2, which means you'd only have enough bandwidth to run dual 1080p displays at 120 Hz.  Your system supports DisplayPort 1.4, which allows more bandwidth, but I can't find any DisplayPort MST hubs that support that.