Too many "greens" and not enough "browns." Read the greens/browns sticky at the top of this Forum (section). For good composting you need a mixture of green (nitrogen heavy/soft/moist) materials and brown (carbon heavy/ hard/ dry) materials. Your leaves (if they are tree leaves, not green plant leaves) are browns, but all the rest is greens. You need probably more browns than greens by volume. Also it sounds like your compost tumbler wasn't well enough aerated. Good compost requires air/ oxygen. Anaerobic composting produces the kind of stinky, slimy mess you are talking about. Does your composter have plenty of air holes? Make sure they didn't get plugged up.
For now I would dump it all out into an outdoor compost pile, add a bunch more browns (the sticky will tell you what they are), mix it all together, and let it sit for a couple more months. If it weren't for the tiny flies (which at a guess are fungus gnats) you could put it back in the tumbler (making sure there's plenty of air access). But with the flies/ gnats, I think before you use the tumbler again, you need to wash it out with a bleach solution and then leave the hatch open to sun and air and let it dry out thoroughly.
Your compost can be rehabilitated!