I read that after LIGO and Virgo interferometers detected gravitational waves signal from a kilonova, many ground based telescopes quickly turns their mirrors towards the spot and "sees" gold and platinum ,etc other heavy elements over the next few days. Is it really possible to directly observe heavy elements being formed consider that the event occurs at a distance of around 130 millions light year away?
No they didn't. The deduction that (very) heavy elements are being produced is somewhat indirect and involves creating a theoretical model of the expanding "fireball" and comparing that model against measurements of brightness vs time at a range of wavelengths and against spectra (mostly in the infrared) as the afterglow faded.
Normally, when we view a stellar spectrum, one can see individual absorption lines that correspond to particular, identifiable elements. In the afterglow of this explosion that is not possible. It is moving so fast that the absorption features that were seen are smeared out by the Doppler effect and the atomic physics required to predict the strength of absorption is too uncertain to make accurate identifications of particular elements.
What is likely though is that the heavy elements, known as lanthanides are providing the large opacity in the "photosphere" of the fireball and are responsible for the rapid fading of the visible signal, the rather more lengthy decline in the infrared and the appearance of extremely broad absorption features that develop in the infrared. The heating of the ejecta by the radioactive decay of very neutron-rich nuclei is also a vital component of explaining the development of the afterglow.