I'm an Italian citizen working for the Italian Government at the UN .

Prior the G1 visa, I was on a B1/B2,J1, E2, G2 changed into G1 visa, all of them issued in Italy.

I always paid taxes in USA; I wish to understand if I'm still considered resident for taxes purposes in the USA.

I'm having also some troubles to understand the art. 19 of the Inernational treaty between Italy and USA

G1 visa holders's compensation from working for the Italian Missions
in the US is taxable only by Italy under article 19(1)(a), which does
not bar the US from taxing them on any other US-source income.
If the G1 visa holder did not come to the US solely to work for the
Italian Missions, the US has the exclusive right to tax under Article
19(1)(b)(ii).

Any help is welcome!

Actually, it's pretty simple.

Your tax status changed when you started working for the Italian government.

The income you receive from the Italian government as salary is EXEMPT from all U.S. taxes (federal and state income taxes, Social Security. Medicare).

However, if you moonlight on a second job and earn extra income while in the U.S. THAT income is subject to all of the U.S. taxes mentioned above.