Re the grep command not returning anything, that seems strange?! Although the command that you posted would almost certainly not work:

grep -r" ^ deb. * Gitlab "/etc/apt/sources.list*

The exact command I posted previously should have worked though (although it's possible if there was a leading space before the 'deb' it might not have; see the explanation on this command at the end if you're interested). Note the exact position of spaces and quotes, and no capital letters, etc). I.e.:

grep -r "^deb .*gitlab" /etc/apt/sources.list*

Regardless, I'm glad that you managed to work it out and that it's working fine again now.

Re your question on the downloaded deb files, by default, those files are cached within /var/cache/apt/archives/. You can have a look at them like this:

 ls -l /var/cache/apt/archives/

If you wish to clean them up, you could manually clean up the deb files and it's unlikely tht ti will cause any issue. However, it's probably better to use apt as it knows exactly what to clean and what to leave. There are 2 possibilities that do slightly different things.

The first is 'autoclean", which clears old packages only (i.e. it will keep the deb files which relate to the currently installed versions). This means that if you wanted to reinstall that particular version, you could do that without having to download them again. It works like this:

apt autoclean

FWIW, in v16.0 (and perhaps v15.x too?) that's what it will do by default everytime you install apps (including the auto security update installs).

The other option is 'clean'. That will remove all cached deb packages. Do that like this:

apt clean

As something of an aside, as I mentioned above, I'll give you a little more info about the grep command that I provided. I.e. this one:

grep -r "^deb .*gitlab" /etc/apt/sources.list*

You can view the general explanation of the command on explainshell.

The part in quotes is regex (in this case "simplified grep regex"). The regex can be explained like this:

^ - means start of the line
deb - the exact characters 'deb' (in that exact order)
 (space) - the exact ' ' character (i.e. a single space)
. - any character
* - repeat previous character 0 or more times
gitlab - the exact characters 'gitlab'  (in that exact order)

In other words, return all lines that start with "deb " and include "gitlab".

The path with the asterisk ('*') at the end (i.e. '/etc/apt/sources.list*') is known as a globbing expression (sort of like a super simple regex provided by the shell). In this case, the asterisk is expanded by the shell to match any character. So it would expand to match any file or folder that starts with '/etc/apt/sources.list' (e.g. will match '/etc/apt/sources.list' and '/etc/apt/sources.list.d' and all the files inside '/etc/apt/sources.list.d/').