I decided to try building an Appimage of a modified version of httpedit by Shordi (on Gambas Farm). The reason is that I use this control to edit html documents in several of my programs and it, in turn, uses the gb.qt5.webkit component. However, it seems that Arch-based distros don't package the qt5.webkit in Gambas, so my apps won't run in EndeavorOS, which I have installed in a new laptop. An Appimage might be a way to run my apps in EndeavourOS.
In principle creating an Appimage should be much easier now that Gambas has an option to create them. I have installed both the appimage-builder Appimage and the appimagetool Appimage in /usr/local/bin/. However, Gambas says that it can't find appimage-builder. So where should I put the appimage-builder Appimage so that Gambas can find it? Or is the problem that Gambas can't use an Appimage of appimage-builder and requires a normal binary of appimage-builder in order to do its magic?
Any help would be appreciated.

 Install appimage builder properly via the package installer. an appimage of the builder copied to /usr/bin is not a proper install.

And WebKit is now depreciated, the program is old but could probably be converted to use webview easily enough.

 Thanks for your feedback, Bruce. I agree that the qt5-webkit is old, but it works and it is well documented in the Gambas help system. I think the qt5-webview route is the best way forward to make my apps more future-proof, but as far as I can see, there is zero documentation for this component in the Gambas help system, so I'm completely in the dark as to how to use it. I managed to make a form with a webview on it and load an html file into it, but it's not editable, and I don't know how to make it editable. I also see that the webview has an ExecJavascript Method, which I assume could be used to manipulate the html content but again, without any documentation, I have absolutely no idea on how to use it. So that's the reason I figured that an Appimage might be the path of least resistance for the time being.
As for appimage-builder, I can see instructions for downloading an Appimage of the app (I've already done that), and some stuff about a Docker image, but I can't see any clear instructions that are comprehensible to my Octogenarian brain on how to download and install a normal binary. So at the moment I really don't have a clear path forward, except to replace EndeavorOS on my newer laptop with a Debian based distro like Debian Testing or maybe Linux Mint 22 when it comes out. Unfortunately the HDMI port on my new laptop, which I need for teaching, doesn't work with Debian Stable based distros and I haven't been able to solve that problem.
But if someone can help with documentation or examples of how to use the qt5-webview as an html editor then I'd love to have a go at using it in place of the qt5-webkit.

SOLVED Re: Problem building an appimage - appimage-builder not found

 After some time I managed to install appimage-builder. When I tried to install it in MX linux I was advised that lots of other packages would be removed. So not wanting to risk possible dependency problems I installed it in a Linux Mint VM. Once that was done I was able to build an appimage from within Gambas 3.19.3.  I tested it successfully in MX23 (Debian Stable), Sparky Rolling (Debian Testing), Manjaro and EndeavourOS, all with a KDE Plasma desktop, and LM21.2 with the XFCE desktop.
The Appimage is a bit slower to start up compared to a normal Gambas executable, but otherwise on my Dell Ryzen there doesn't seem to be much difference in speed once the app is up and running.
And one more thing … the AppImage is 63 Mb, compared to 2.9 MB for the same Gambas executable. I  guess that's the penalty one has to pay for cross-distro compatibility.

For others, there is a guide to build Gambas into Appimage:
Gambas3 Application - appimage-builder 1.0.0 documentation