>>40175468>can't trade anymoreTo trade Heart Gold/Soul Silver, you can trade from HG/SS to Black/White/B2/W2. From there, you can then trade it to Pokemon X. It's a one way trade.
It's honestly convoluted and ridiculous, but here's a website that goes over everything. Please note that many people who hack Pokemon will use a similar process to this, because it hides the fact that it's hacked pretty well.
https://www.thonky.com/pokemon/trade-from-heartgold-to-x