gg5: a shed; a (storage) barn; an outbuilding; an outhouse.

"outhouse" is synonymous with "outbuilding" in BrE but I don't mind omitting it here.

@@ -18 +18 @@
-<gloss>barn</gloss>
+<gloss>barn (for storage)</gloss>
@@ -19,0 +20 @@
+<gloss>outbuilding</gloss>

https://ja.wikipedia.org/wiki/納屋
納屋（なや、英語: Barn）とは、主に別棟に建てられた物置小屋のことである。特に農家では農作物や農機具などを収納しておく小屋を指す。漁村では、舟や網を収納する小屋、もしくは若い漁師を起居させる網元が用意した小屋の事である。

Mostly barn, but here's a possible mistake below:
https://context.reverso.net/translation/japanese-english/納屋
Obvioulsly E->J
Morning 5 September, the siege began when Chief Winamac's forces attacked two soldiers returning from an outhouse.
9月5日朝、ウィナマック酋長の部隊が納屋から砦に戻る兵士2人を攻撃したときに包囲戦が始まった。

Now, is that outhouse a bathroom? (As an American reader, that's my understanding).  Is it still a bathroom in Japanese?

daijs has 4 senses, none of them look like "bathroom" to me.
https://kotobank.jp/word/納屋-589913

This is more to do with English than Japanese.  I think "outhouse" has too strong of a "tiny wooden outdoor bathroom" connotation in modern English to be useful here(or rather, it risks being strongly misleading here).  Unless 納屋 can also have this connotation, which I don't think it does, I think we're better off dropping it.

@@ -17,0 +18 @@
+<gloss>barn</gloss>
@@ -19,2 +19,0 @@
-<gloss>barn</gloss>
-<gloss>outhouse</gloss>