Rust convertor and paint for metal gate

Try any large Brico.

"better than stating the blooming obvious"

Surprisingly CC_R, although I know a lot, unfortunately I don't know the aisle arrangement and shelf layout of every Brico in Belgium so I can't guide you at that level of detail. Just go along and ask for it, they'll have it in every shop. I don't know the exact phrase in French, but I'd start with "peinture antirouille" (no idea what you would try in Flemish I'm afraid).

And for your further question about cost, you're unlikely to get it significantly cheaper than Brico, unless you try shopping in Holland, Germany or the U.K. where you can buy this sort of stuff at perhaps half the price.