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This article will help with that: https://www.gardeningknowhow.com/edible/fruits/figs/fig-propagation.htm

There's no such thing as figlia plant, but perhaps you mean some type of ficus, most likely a fiddle leaf fig? That has large leaves - around 6 - 10" long and shaped roughly like a fiddle; possibly also a ficus decora, known as rubber tree, that has large thick darkgreen leaves. Those two would be the most likely to have only one main trunk with no side branches. Given that the plant is healthy and growing, you can simply cut off the top top few inches, and it might well start to branch. If you send in a picture to this address, we might be able to give you more information. It's alway important to know the correct name for any plant you wish to be successful with.
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Sounds like your Fig might be suffering from Fig Rust and/or Leaf Blight. These articles may help you identify and treat it: https://www.gardeningknowhow.com/edible/fruits/figs/figs-disease-rust.htm

https://www.gardeningknowhow.com/edible/fruits/figs/common-fig-tree-diseases.htm

The gnats that you see happily hanging around your tree are likely fungus gnats or soil gnats, and they generally don't do any harm, but the larvae of certain types can damage the plant when they feed on the roots. This article may help you get these tiny bugs under control:

https://www.gardeningknowhow.com/plant-problems/pests/insects/fungus-gnats.htm

Avoid moving your plant around; they are finicky and don't like change.

Aim for a nice bright location, an East facing window is ideal.

Misting during dry spells or winter can help.

Avoid drafts from air conditioners, they actually like warm temperatures.

To water properly, wait until the top inch of soil is dry. Then, thoroughly drench until water comes out the bottom of the pot, and leave it to slowly dry out again.

Even though you seem to be watering correctly, it would appear that this has a fungal infection. Apply wettable sulfur to the soil, along with some dolomitic lime. This will help kill off the infection.

Sometime during it's life it has contracted a severe fungal infection. It is going to take quite a bit of effort on your part to be able to save this.

The first thing to be done is to take is out of its soil, and completely wash off the roots VERY CAREFULLY.

After this, you can soak it in a half and half mix of water and peroxide. You will want to do this for 2 to 3 minutes. Then replant it into fresh soil with plenty of drainage. I recommend a good potting soil with a good mixture of perlite.

Top dress with a coating of dolomitic lime, and wettable sulfur according to the packages. As you water in, these will condition the soil, and kill off any remaining infection.

It is going to take a little while, but it CAN recover.