I'm a new user, but I don't understand why apps like LibreOffice and Gimp on AppCenter are far away from the current version. Every time I want something current I have to go to the developer's own site and it's very inconvenient :/
That might be because those non-curated apps come from the Ubuntu apt repositories. Elementary OS is based on a Ubuntu LTS release which in turn is based on Debian Testing at the time of the release.
At the time of writing the Elementary OS 5.x is based on Ubuntu 18.04 LTS, released in October 2018, which came with the GNOME 3.28 desktop environment and associated apps like GIMP that you mention. This may explain the outdated, non-curated apps.
Having said that: I do see the latest versions of many apps in the AppCenter however as well as outdated ones. GIMP has both 2.10.14 (current release) and the 2.8.22 versions at the time of writing. The more up-to-date one must be a Flatpak from Elementary's custom Flatpak-repositories. The flatpaks are usually up-to-date with the latest version of the developers.
The same goes for LibreOffice: the latest version is available in the AppCenter as is an older version.
If you are not seeing those up-to-date apps in the AppStore first check if you are using the latest version of Elementary OS. You need to be on Elementary OS 5.1 (Hera) to get the flatpaks in the AppCenter. If so, try updating the repositories. Right-click on the AppCenter icon and "Check for updates" or update the repositories from a terminal (Super+T):
sudo apt update
then check the AppCenter again.
You can also get flatpaks from Flathub but that is not the "official" way to get apps in Elementary OS.
I recently learned that the flatpaks that I see in the AppCenter are actually from Flathub. They got in there because I once "sideloaded" an app from Flathub.org which then adds that repository to Elementary's store.
Flatpak has current versions of the apps you mentioned and a lot more and support in elementary is seamless. See the install instructions here. Instead of Thunderbird or Evolution as mentioned in the linked guide install Gimp or LibreOffice to enable support.
Elementary does not yet have their own Flatpak repo hence the reason why Flatpak apps do not show up in the AppCenter by default. A custom repo was their original intent and may be something introduced in the future (as documented in April, 2019). As for now elementary created the sideload application (see the more recent blog post) which will allow you to install files downloaded from other repositories (flathub is the biggie, see the official flatpak install guide submitted by the elementary OS team) by downloading and installing one .flatpakref file which will add all applications from that repo to the AppCenter.