Synthetic organic reactions involving gaseous reagents are frequently avoided in the research lab for practical and safety reasons. Nevertheless, reactive gases often represent convenient building blocks with high potential for atom economy. This review article aims at highlighting ex situ gas generation methods as safe and easily executable procedures for organic synthesis. Especially the advent of two-chamber type reactors has been responsible for the renewed interest in gas–liquid biphasic transformations. With the exemption of carbon monoxide, a discussion is provided on the scientific work from the last decade which employs ex situ generated gaseous reagents, produced on-demand and in a place other than the reaction mixture itself. Features of these protocols which are especially appealing to a laboratory bench-scale context will be underscored, with an emphasis on safety and synthetic diversity allowed by the gas-mediated reactions.