As an aside, I'm curious why you thought they were Japanese specifically, as opposed to Chinese.

They are hausmarken, singular hausmarke. The concept was rather like a coat of arms, but they were merchants' marks, with some similarity to makers' marks, printers' marks, and such. They tended to be geometrical figures of lines. These look like a common type, comprising joined line segments, often with a vertical post (but by no means universally), with various angled (or not angled) segments.

They were used in Germany, and north and east (I think Polish herby visually resembled them somewhat). And you mentioned that these were in "Elzas", rendered in English as Alsace, which of course has a strong German influence.

The source I've seen is Erasimierz Waspanieski, "Hausmarken: Burger Heraldry of the German Renaissance", but it's pretty obscure: the proceedings of the 1990 Known World Heraldic Symposium, an annual gathering in the Society for Creative Anachronism. I can't find my copy at the moment, but if you have a strong desire, I can ask a friend to scan a copy of the article. This appears to be a source from the 13th Century; I don't know the source, but they certainly resemble the ones I've seen.

Assuming it's not some kind of cross I'm unfamiliar with, there are a couple things. First it looks like something done by someone who didn't actually write the language, if it is in fact Chinese characters. The one on the right looks like 未, but the on on the left is missing the top half. You can see the stone has eroded considerably. The bottom kinda looks like 太 with an extra line, or could be something like 术, but again that's likely only half of the character and again it looks like someone who didn't know exactly what they were doing was the one to make it.

Do you have any pictures other than this? Engraved stone is hard to photograph well to show the levels, and you're really beholden to the lighting, but it might help all the same.

As for my suspicion it wasn't a Chinese/Japanese person behind this — aside from the character shapes — it was quite popular in Europe to incorporate Eastern motifs in art. Delftware was an attempt to replicate the Chinese blue and white pottery styles, for example. This lasted centuries. If you're interested in more examples, see the Wikipedia article on Chinoiserie. The famous Monet paintings of the bridge in his garden are of a Japanese-styled bridge.

It's thus entirely possible that what you're seeing here is a Frenchman's attempt at adding an Asian flair to his gate, possibly not knowing or much caring what the characters said, if anything, and instead just making an attempt at something that looked Asian without actually being a thing. You see this in tattoos today, where people have "Chinese characters" that are neither Chinese nor actual characters, but look like they are to someone who wouldn't know otherwise.

So, that's what I think is going on here, but I'd be interested to see more photos if you have them.

tl;dr: I believe what you're looking at is just Chinoiserie and nothing more.