They do also sell keys, but most of the time the Steam gift is way cheaper.

For example the Garrys Mod Steam gift goes for ~9€ [1] while the key goes for ~16€ [2]

[1]https://www.g2a.com/garrys-mod-steam-gift-global-i1000000439... [2]https://www.g2a.com/garrys-mod-steam-key-global-i10000004391...