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'"

I can confirm.

Operating System: KDE neon User Edition
KDE Plasma Version: 6.4.2
KDE Frameworks Version: 6.15.0
Qt Version: 6.9.0
Kernel Version: 6.11.0-29-generic (64-bit)
Graphics Platform: Wayland

Those file types are not archive in a sense, they don't have embedded file hierarchy and tgz do.
They are the content of a file that is compressed.

As such the issue is not that dolphin doesn't open them but tries to open them in the first place when it should launch ark instead.

A possibly relevant merge request was started @ https://invent.kde.org/frameworks/kio/-/merge_requests/2007

Git commit e4f0bd934cef21c9f6031b25b1ec60c288f37daf by Méven Car.
Committed on 28/09/2025 at 12:43.
Pushed by meven into branch 'master'.

kprotocolmanager: don't consider stream protocol for ArchiveMimetype

Since users of this function will want to open the archive with the list
protocol logic but that is impossible for stream file, restrict the
protocol to take into consideration to filesystem input ones.

M  +6    -3    src/core/kprotocolmanager.cpp
M  +2    -0    src/core/kprotocolmanager.h

https://invent.kde.org/frameworks/kio/-/commit/e4f0bd934cef21c9f6031b25b1ec60c288f37daf