Note that when you say "install CUDA", it's not clear if you mean you installed the CUDA Toolkit/SDK, or installed the driver. MATLAB only needs the driver installed and running to use the GPU, the toolkit/SDK is not necessary. So I'll assume that something is wrong with your driver. 
- Run "nvidia-smi" in a command line and see if the GPU is reported correctly. It should look something like this:
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 440.33.01    Driver Version: 440.33.01    CUDA Version: 10.2     |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  Tesla K80           Off  | 00000000:04:00.0 Off |                    0 |
| N/A   37C    P0    57W / 149W |      0MiB / 11441MiB |      0%      Default |
+-------------------------------+----------------------+----------------------+
|   1  Tesla K80           Off  | 00000000:05:00.0 Off |                    0 |
| N/A   43C    P0    70W / 149W |      0MiB / 11441MiB |     51%      Default |
+-------------------------------+----------------------+----------------------+
                                                                               
+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID   Type   Process name                             Usage      |
|=============================================================================|
|  No running processes found                                                 |
+-----------------------------------------------------------------------------+
- If nvidia-smi isn't working, the driver is not running at all. You should try reinstalling the latest driver, and also check that your system is finding the GPU. To see if the system sees the GPU, use 
% lspci | grep -i nvidia
04:00.0 3D controller: NVIDIA Corporation GK210GL [Tesla K80] (rev a1)
05:00.0 3D controller: NVIDIA Corporation GK210GL [Tesla K80] (rev a1)
- If the GPU shows up in lspci but nvidia-smi doesn't find anything, your driver is not working at all -- reinstall it.     
- If it's working, check the permissions on your GPU device, it should look something like the following. I've seen cases where the GPU is only accessible to root.
% ls -l /dev/nv*
crw-rw-rw- 1 root root 195,   0 Jan 16 16:17 /dev/nvidia0
crw-rw-rw- 1 root root 195,   1 Jan 16 16:17 /dev/nvidia1
crw-rw-rw- 1 root root 195, 255 Jan 16 16:17 /dev/nvidiactl
crw-rw-rw- 1 root root 243,   0 Jan 16 16:17 /dev/nvidia-uvm
crw-rw-rw- 1 root root 243,   1 Jan 16 16:17 /dev/nvidia-uvm-tools
- If the permissions are wrong, on some driver versions you can set NVreg_DeviceFileMode=0660 to NVreg_DeviceFileMode=0666. The conf file you edit is in /etc/modprobe.d, it's generally called something with "nvidia" in it, for example 50-nvidia.conf. Note that this setting may be depricated, I haven't seen it in a while.
Check and see if the nvidia module is loaded, and that "nouveau" is not. 
% lsmod | grep nvidia
nvidia_uvm            917504  0
nvidia_drm             45056  2
nvidia_modeset       1110016  1 nvidia_drm
nvidia              19894272  2 nvidia_modeset,nvidia_uvm
drm_kms_helper        155648  2 mgag200,nvidia_drm
drm                   360448  10 mgag200,ttm,nvidia_drm,drm_kms_helper
ipmi_msghandler        49152  2 nvidia,ipmi_si
% lsmod | grep nouveau
(there should be no output)
If nouveau is loaded, you need to blacklist it by adding a file in /etc/modprobe.d that says something like this. The driver installation process usually does this (I have a file called "nouveau-blacklist.conf"), but that might have been removed for some reason. You may not need the lvm-nouveau line, depending on what you are running.
blacklist nouveau
blacklist lvm-nouveau
This will require a reboot to realize the blacklisting.