Question for Linux gurus. Why won't gdm3 start?

Question

linux

Good day.
I bought a computer (not a laptop), loaded the last one on a Debian disc, installed it, then installed non-free drivers for the video card (ati radeon), after installation gdm3 does not start, that is, after launch - a black screen. If I log in to another console (ctrl+alt+f1) as a normal user and type startx, I go into graphics.
Question to the Linux guru, why doesn't gdm start, what am I doing wrong?
System parameters:

# uname -a      4.4K Jan 12 22:24 Xorg.4.log
Linux moipk 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u2 (2016-01-02) x86_64 GNU/Linux

# lspci 
00:00.0 Host bridge: Advanced Micro Devices, Inc. [AMD] RS780 Host Bridge
00:02.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780 PCI to PCI bridge (ext gfx port 0)
00:04.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780/RS880 PCI to PCI bridge (PCIE port 0)
00:11.0 SATA controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 SATA Controller [IDE mode]
00:12.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:12.1 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0 USB OHCI1 Controller
00:12.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB EHCI Controller
00:13.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:13.1 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0 USB OHCI1 Controller
00:13.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB EHCI Controller
00:14.0 SMBus: Advanced Micro Devices, Inc. [AMD/ATI] SBx00 SMBus Controller (rev 3c)
00:14.1 IDE interface: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 IDE Controller
00:14.2 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] SBx00 Azalia (Intel HDA)
00:14.3 ISA bridge: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 LPC host controller
00:14.4 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] SBx00 PCI to PCI Bridge
00:14.5 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0/SB8x0/SB9x0 USB OHCI2 Controller
00:18.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 15h Processor Function 0
00:18.1 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 15h Processor Function 1
00:18.2 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 15h Processor Function 2
00:18.3 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 15h Processor Function 3
00:18.4 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 15h Processor Function 4
00:18.5 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 15h Processor Function 5
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Oland XT [Radeon HD 8670 / R7 250]
01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800 Series]
02:00.0 Ethernet controller: Qualcomm Atheros QCA8171 Gigabit Ethernet (rev 10)

# tail -n 15 -f /var/log/Xorg.0.log
[    16.397] (II) fglrx(0): Modeline "1024x768"x0.0   75.00  1024 1048 1184 1328  768 771 777 806 -hsync -vsync (56.5 kHz e)
[    16.397] (II) fglrx(0): Modeline "1024x768"x0.0   65.00  1024 1048 1184 1344  768 771 777 806 -hsync -vsync (48.4 kHz e)
[    16.397] (II) fglrx(0): Modeline "832x624"x0.0   57.28  832 864 928 1152  624 625 628 667 -hsync -vsync (49.7 kHz e)
[    16.397] (II) fglrx(0): Modeline "800x600"x0.0   49.50  800 816 896 1056  600 601 604 625 +hsync +vsync (46.9 kHz e)
[    16.397] (II) fglrx(0): Modeline "800x600"x0.0   50.00  800 856 976 1040  600 637 643 666 +hsync +vsync (48.1 kHz e)
[    16.397] (II) fglrx(0): Modeline "1152x864"x0.0  108.00  1152 1216 1344 1600  864 865 868 900 +hsync +vsync (67.5 kHz e)
[    16.397] (II) fglrx(0): Modeline "1280x720"x60.0   74.48  1280 1336 1472 1664  720 721 724 746 -hsync +vsync (44.8 kHz e)
[    16.397] (II) fglrx(0): Modeline "1280x800"x0.0   83.50  1280 1352 1480 1680  800 803 809 831 -hsync +vsync (49.7 kHz e)
[    16.397] (II) fglrx(0): Modeline "1280x1024"x0.0  108.00  1280 1328 1440 1688  1024 1025 1028 1066 +hsync +vsync (64.0 kHz e)
[    16.397] (II) fglrx(0): Modeline "1440x900"x0.0  106.50  1440 1520 1672 1904  900 903 909 934 -hsync +vsync (55.9 kHz e)
[    16.397] (II) fglrx(0): Modeline "1600x900"x60.0  119.00  1600 1696 1864 2128  900 901 904 932 -hsync +vsync (55.9 kHz e)
[    16.397] (II) fglrx(0): Modeline "1680x1050"x0.0  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync (65.3 kHz e)
[    25.768] (II) AIGLX: Suspending AIGLX clients for VT switch
[    25.768] (II) fglrx(0): Backup framebuffer data.
[    25.789] (II) fglrx(0): Backup complete.

# dpkg --list | grep fglrx
ii  fglrx-atieventsd                      1:14.9+ga14.201-2                    amd64        events daemon for the non-free ATI/AMD RadeonHD display driver
ii  fglrx-control                         1:14.9+ga14.201-2                    amd64        control panel for the non-free ATI/AMD RadeonHD display driver
ii  fglrx-driver                          1:14.9+ga14.201-2                    amd64        non-free ATI/AMD RadeonHD display driver
ii  fglrx-modules-dkms                    1:14.9+ga14.201-2                    amd64        dkms module source for the non-free ATI/AMD RadeonHD display driver
ii  glx-alternative-fglrx                 0.5.1                                amd64        allows the selection of FGLRX as GLX provider
ii  libfglrx:amd64                        1:14.9+ga14.201-2                    amd64        non-free ATI/AMD RadeonHD display driver (runtime libraries)
ii  libfglrx-amdxvba1:amd64               1:14.9+ga14.201-2                    amd64        AMD XvBA (X-Video Bitstream Acceleration) runtime libraries
ii  libgl1-fglrx-glx:amd64                1:14.9+ga14.201-2                    amd64        proprietary libGL for the non-free ATI/AMD RadeonHD display driver
ii  xvba-va-driver                        0.8.0-9                              amd64        XvBA-based backend for VA API (AMD fglrx implementation)

moipk:/var/log# grep EE /var/log/Xorg.0.log
  (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
moipk:/var/log#

Answer 1

Debian already has systemd
look at what gdm writes in journalctl
try to restart gdm from another console and see the logs

Answer 2

see dmesg
and just do, why on the laptop debian? Is this some kind of masochism?