I read [Lubuntu] uses the same repositories as Ubuntu
You read right. The difference between Ubuntu and Lubuntu is the desktop environment, which is a collection of packages. All flavors of Ubuntu use the same software repositories, and therefore have the same packages available to install, although they have different applications installed by default, particularly GUI software that makes up the DE.
When trying Lubuntu from a disk [...] the "shop" (app market) did not show GIMP or OpenShot in search
I think this is because those two packages are in the Universe repository, which was not available in the live environment. The Universe repository should be available after installation, but if it isn't, you can easily enable it:
How do I enable the "Universe" repository?
After that, GIMP and OpenShot should appear when you search in the AppMarket. If for some reason they don't, you can install them using APT:
sudo apt install gimp openshot