GIMPShop is not a thing. The actual GIMPShop project (a fork of GIMP aiming to be similar to Photoshop) was abandoned ~2006. There are some old & untrustworthy tarballs/.deb packages floating around but I checked one out recently and it was completely broken.
That site you link to is basically a scam. They offer an installer that downloads the normal GIMP from the official sources but is also some kind of adware. As their site even admits:
Gimpshop.com is distributing the original unmodified software,
obtained directly from GIMP's repository website, and does not modify
it in any way. These installers comply with the original software
manufacturer's policies and terms & conditions. DownloadAdmin™ is an
install manager, which manages the installation of your chosen
software. In addition to managing your download and installation,
DownloadAdmin™ will offer free popular software that you may be
interested in. You are not required to install any additional software
to complete your installation of your selected software.
I asked a question about this on Open Source SE, and I believe the site is violating the GPL license of GIMP.
@edwinksl also pointed me to this relevant post and some bad reviews of the site here
Do not attempt to download anything from the GIMPSHOP site
Install gimp safely from repositories:
sudo apt install gimp