qtractor-1.4.0-6.1.x86_64.AppImage

TimGM6mb.sf2 is not a plugin, it's a soundfont that can be loaded and played by an sf2 player instrument plugin that you have to load, eg. fluidsynth-dssi or Calf Fluidsynth.

hi,
you first need to install the plugins; qtractor doesn't come with any at all, so you better search your distro package manager for some.
cheers

i recently switched back to Linux Mint as well.
If you have no soft synths, these will be older versions but quickest way to get them.
just type in your terminal with root access (assuming you have the space),
apt install zynaddsubfx yoshimi amsynth add64 synthv1 padthv1 samplv1 drumkv1 mda-lv2 calf-plugins lsp-plugins qsynth fluidsynth carla calf-plugins avldrums.lv2
these will give you basic synths and effects as well as an mda piano and epiano and avldrums
i personally don't use soundfont files so i can't say for certain or how, but i heard carla can load soundfont files.
then just go in Qtractor, add plugin, and search for em. (they should be automatically added to /usr/lib/lv2 or /usr/local/lib/2 which Qtractor detects out the box afaik)
you can also find and download .deb or archive files for Helm, Dexed, Surge-XT, TAL plugins etc, and copy the extracted files to the aforementioned lv2 folders or manually add their path in Qtractor settings.