Gasket alternative, silicon Options

[FSX_Celica94GT]

I was talking to one of my friends who is a mechanic and i told him i got a header. I told him i needed gaskets, another catalytic converter, and a d/p made for it to put it on. He said that i didnt need gaskets, he said i could use this sort of silicon that he uses to seal the pipes on his car (68 dodge dart, put a new v8 in it himself) and said he hasnt had any problems w/ it, what do you guys think? is it safe?