I also have this issue (though I notice it is tagged with [radeon]  and I have nvidia). I am not positive but I believe it started when I did the in-place upgrade to 18.10 from 18.04. SDDM does not have the same issue so it appears to be GDM specific.

Output from lspci -k for reference as requested of the original poster above:


#### lspci -k ####

00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor Host Bridge/DRAM Registers (rev 07)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: skl_uncore
00:01.0 PCI bridge: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor PCIe Controller (x16) (rev 07)
	Kernel driver in use: pcieport
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 530 (rev 06)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: i915
	Kernel modules: i915
00:04.0 Signal processing controller: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor Thermal Subsystem (rev 07)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: proc_thermal
	Kernel modules: processor_thermal_device
00:14.0 USB controller: Intel Corporation 100 Series/C230 Series Chipset Family USB 3.0 xHCI Controller (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: xhci_hcd
00:14.2 Signal processing controller: Intel Corporation 100 Series/C230 Series Chipset Family Thermal Subsystem (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: intel_pch_thermal
	Kernel modules: intel_pch_thermal
00:15.0 Signal processing controller: Intel Corporation 100 Series/C230 Series Chipset Family Serial IO I2C Controller #0 (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: intel-lpss
	Kernel modules: intel_lpss_pci
00:15.1 Signal processing controller: Intel Corporation 100 Series/C230 Series Chipset Family Serial IO I2C Controller #1 (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: intel-lpss
	Kernel modules: intel_lpss_pci
00:16.0 Communication controller: Intel Corporation 100 Series/C230 Series Chipset Family MEI Controller #1 (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: mei_me
	Kernel modules: mei_me
00:17.0 SATA controller: Intel Corporation HM170/QM170 Chipset SATA Controller [AHCI Mode] (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: ahci
	Kernel modules: ahci
00:1c.0 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #1 (rev f1)
	Kernel driver in use: pcieport
00:1c.1 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #2 (rev f1)
	Kernel driver in use: pcieport
00:1d.0 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #9 (rev f1)
	Kernel driver in use: pcieport
00:1d.4 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #13 (rev f1)
	Kernel driver in use: pcieport
00:1d.6 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #15 (rev f1)
	Kernel driver in use: pcieport
00:1f.0 ISA bridge: Intel Corporation HM170 Chipset LPC/eSPI Controller (rev 31)
	Subsystem: Dell XPS 15 9550
00:1f.2 Memory controller: Intel Corporation 100 Series/C230 Series Chipset Family Power Management Controller (rev 31)
	Subsystem: Dell XPS 15 9550
00:1f.3 Audio device: Intel Corporation 100 Series/C230 Series Chipset Family HD Audio Controller (rev 31)
	Subsystem: Dell Sunrise Point-H HD Audio
	Kernel driver in use: snd_hda_intel
	Kernel modules: snd_hda_intel
00:1f.4 SMBus: Intel Corporation 100 Series/C230 Series Chipset Family SMBus (rev 31)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: i801_smbus
	Kernel modules: i2c_i801
01:00.0 3D controller: NVIDIA Corporation GM107M [GeForce GTX 960M] (rev a2)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: nvidia
	Kernel modules: nvidiafb, nouveau, nvidia_drm, nvidia
02:00.0 Network controller: Broadcom Inc. and subsidiaries BCM43602 802.11ac Wireless LAN SoC (rev 01)
	Subsystem: Dell BCM43602 802.11ac Wireless LAN SoC
	Kernel driver in use: brcmfmac
	Kernel modules: brcmfmac
03:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. RTS525A PCI Express Card Reader (rev 01)
	Subsystem: Dell XPS 15 9550
	Kernel driver in use: rtsx_pci
	Kernel modules: rtsx_pci
04:00.0 Non-Volatile memory controller: Toshiba America Info Systems NVMe Controller (rev 01)
	Subsystem: Toshiba America Info Systems NVMe Controller
	Kernel driver in use: nvme
	Kernel modules: nvme

#### end of lspci -k output ####