Hello !

It's quite a standard procedure. Just write to Gog's support channel about it and they'll give you an adress to upload the *.Iso file corresponding to your CD - which you'll have to convert to such a format by yourself. I don't remember wether or not they do accept physical gifts through the post office though...
It may take some time for them to actually make use of it, but they usually welcome such initiatives (and so does the french community too !), so go ahead ^^...