I found the reason of this problem when I updated my VirtualBox from version 5.1 to 5.2. This new version comes with a great tool that enables us to resize a VDI disk in an easy way (this tool is inside the option Global Tools).
After accessing it, I found out that my VDI disk have been resized to 23000 MB, as I wrote in the question, using the command
vboxmanage modifyhd VDI_FILE_PATH --resize 20000, as I set it before, but (I don't know why), Virtualbox keeps a VDI with a name in Hexadecimal, as a subdisk of your VDI. This subdisk was only 8GB of total space (because of it, Gparted didn't recognize the resize I made).
At this time, I use the new interface of the updated VirtualBox and set it to 23 GB as the parent disk. Now, I ran the Gparted again in the VM, and now my disk is 23GB.
So if you guys, like me, are facing this issue (not able to properly resize the VDI), check if you VirtualBox is updated to 5.2 (or above) and increase the size of the VDI (and probably the subdisk too) in the Global Tools option
Note: If I go to Settings, the Virtualbox only shows one VDI (the parent), the subdisk is only shown in Global Tools option.