Braggadar: Nope, no gift cards or certificates.
People will argue it doesn't pay to put in a system like that, but i'd disagree: gift cards are quite popular over here in Australia. Parents & Grandparents buy their kids Steam gift cards
I suppose that could be part of the problem.
Because there are games which are not suitable for kids, and the rating system is not universal, it would be very tricky to make sure that the person using the card is old enough to buy some game and that the correct rating system is applied.
From the customer's perspective you could argue that even today you can buy gift codes for rated games and then gift them to kids, but there is a big difference whether some adult buys a game and then gifts it to a kid, or if a kid can buy any game they choose for themselves.
I have no idea how Steam does the age and locale verification in those cases, but I can see many reasons why GOG doesn't even attempt to do it.
Of course historically speaking GOG had a catalogue that wasn't the most attractive to kids, and I suppose that's the situation even today.