[H] I trade with someone for a gift. few days later the gift disappered [W] what should i do

Hey. It's been revoked probably. The reason for that is probably bank canceled payment to Steam or fraudulent use of credit/debit card.
You should contact Steam (and as I see you did) and wait for their response. Steam support is known as probably the most retarded support ever seen. They just copy/paste and it takes them weeks to respond. So don't rush, Valve's support is bad. You will probably get your gift returned and the trader banned. Good luck :)