I am unable to replicate that error on git-master. Double clicking a .gz file in Dolphin opens the file in Ark, the default compressed file application. This is likely a misconfiguration of your system.

There are a couple of things you can check.
- Make sure Ark or another application is installed to be able to open .gz, .zip and other compressed files. 
- Try opening Ark / your compressed file app and open the file from there
- Go into Dolphin Settings - Context Menu - is "'Compress' service menu" checked?

(In reply to TraceyC from comment #1)
> I am unable to replicate that error on git-master. Double clicking a .gz
> file in Dolphin opens the file in Ark, the default compressed file
> application. This is likely a misconfiguration of your system.
> 
> There are a couple of things you can check.
> - Make sure Ark or another application is installed to be able to open .gz,
> .zip and other compressed files. 
> - Try opening Ark / your compressed file app and open the file from there
> - Go into Dolphin Settings - Context Menu - is "'Compress' service menu"
> checked?

Hi,
I did some tests:

- preparing files for the test:

$ mkdir -pv /tmp/tempdir/test
$ cd /tmp/tempdir
$ date > test/file.txt
$ tar cf 1.tar test/file.txt
$ tar --gzip -cf 2.tgz test/file.txt
$ tar --lzma -cf 3.tar.xz test/file.txt
$ tar --lzop -cf 4.tar.lzop test/file.txt
$ lzop < test/file.txt > 5.lzop
$ lzma < test/file.txt > 6.lzm
$ gzip test/file.txt && mv test/file.txt.gz 7.gz
$ rmdir -v test

move files to "$HOME/Desktop"

$ mv -v 1.tar  2.tgz  3.tar.xz  4.tar.lzop  5.lzop  6.lzma  7.gz "${HOME:?}/Desktop"
$ cd "${HOME:?}/Desktop"
$ rmdir -v /tmp/tempdir
$ ls

1.tar  2.tgz  3.tar.xz  4.tar.lzop  5.lzop  6.lzma  7.gz

open from desktop:
 1.tar      -> open ark - OK
 2.tgz      -> open ark - OK
 3.tar.xz   -> open ark - OK
 4.tar.lzop -> open ark - OK
 5.lzop     -> open ark - OK
 6.lzma     -> open ark - OK
 7.gz       -> open ark - OK

open with Dolphin (flag "Settings->View->Open archive as folder" ENABLED):
 1.tar      -> view folder - OK
 2.tgz      -> view folder - OK
 3.tar.xz   -> view folder - OK
 4.tar.lzop -> open ark    - OK
 5.lzop     -> open ark    - OK
 6.lzma     -> Invalid protocol 'lzma' - ERROR
 7.gz       -> Invalid protocol 'gzip' - ERROR

open with Dolphin (flag "Settings->View->Open archive as folder" DISABLED):
 1.tar      -> open ark    - OK
 2.tgz      -> open ark    - OK
 3.tar.xz   -> open ark    - OK
 4.tar.lzop -> open ark    - OK
 5.lzop     -> open ark    - OK
 6.lzma     -> open ark    - OK
 7.gz       -> open ark    - OK

mimelist content:
$ grep "ark.desktop" "$HOME/.config/mimeapps.list"
application/x-compressed-tar=ark.desktop;org.kde.ark.desktop;xarchiver.desktop;
application/x-gzip=ark.desktop;org.kde.ark.desktop;xarchiver.desktop;
application/x-tar=org.kde.ark.desktop;xarchiver.desktop;
application/x-tar-gz=org.kde.ark.desktop;xarchiver.desktop;
application/x-tarz=org.kde.ark.desktop;xarchiver.desktop;
application/x-tgz=org.kde.ark.desktop;xarchiver.desktop;
application/x-xz-compressed-tar=org.kde.ark.desktop;org.kde.kate.desktop;xarchiver.desktop;
application/zip=org.kde.ark.desktop;
application/gzip=org.kde.ark.desktop;xarchiver.desktop;

so the problem occurs when the "Open archive as folder" is enabled

Thanks for the detailed testing! I am able to replicate the error on Dolphin built from git-master with these steps
Note that xz is the successor to lzma, and my distro doesn't have support for files with the lzma extension, so I saved the lzma file with the xz extension

**System setup**
- Dolphin Settings - Enabled "Open archive as folder"

date > file.txt
lzma < file.txt > 6.xz
gzip file.txt && mv file.txt.gz 7.gz

1. Open Dolphin and try to open the two test files

Result: "Invalid protocol 'gzip'" or "Invalid protocol 'xz'"