Install synaptic

The Ubuntu Software (Center) does not list all software available in repositories or installed. This is particularly true for command-line utilities. Another GUI alternative to Ubuntu Software is synaptic also known as Synaptic Package Manager. You should be able to find in in Ubuntu Software. If not, install using the following command:

sudo apt install synaptic 

Open synaptic

Open synaptic as you usually open any GUI software. It will prompt you to enter your password before it opens.

On the left panel you can select Installed to see all the installed software in your system:

The green squares indicate that a software is installed. If you want to see if a specific software, such as gnome-software is installed, click on the big search button and type the name of the software. You can limit the search to just Names or Names and Description (if you want to search for a keyword, rather than the exact name). You will get something like this:

As you can see above gnome-software and gnome-software-commonare installed and other packages with similar names are not installed, as they don't have the green square next to them.

Hope this helps

Since gnome-software/ubuntu-software isn't showing your installed apps you should try listing your installed apps in Terminal!

Just type this command in the terminal "apt list --installed"

Hope this has helped!

:D

EDIT 1a: Please note that this command shows more than what gnome-software/ubuntu-software shows! There may be filters for this command but I don't know!