I agree with @TizerorFizz, GCSE language can only be a starting point. Of course even a 9 at GCSE German does not mean you can read Goethe or that you are bilingual.
But a student who has recently achieved a good GCSE grade should be able to hold a conversation in German about their hobbies, their family, their home town, social issues such as the environment and poverty. They should be able to read things in Germany, chat to staff in hotels, restaurants and museums, maybe even pick up a newspaper and tell you what the story is about.
That's all tremendously valuable in my view. And if the student wants to go on with MFL, to A level, or to go to the country, or to take some speaking classes in the evening, well GCSE is a great start.
@BananaPB you don't say when your DC took their MFL GCSEs, but if it was before 2018 then that makes a difference too. I am no apologist for the Tories, but the new spec MFL GCSE is much more rigourous, discouraging rote learning and supporting students to actually acquire language skills such as translation and spontaneous speaking.
Yes, if you don;t use it you will forget it. I took chemistry O level (old gimmer) and pretty much all I can remember is the difference between HCl and H2O - but that's because I did nothing else with it. It was still worth me studying it. Maths GCSE doesn't turn you into a top level mathematician; French GCSE doesn't make you an interpreter. But it's a start, and even if that's all, it's as worthwhile as any GCSE.