YOU SAID:

I didn't know that you went to gym too.

INTO JAPANESE

私はあなたもジムに行ったことを知りませんでした。

BACK INTO ENGLISH

I didn't know you went to the gym either.

INTO JAPANESE

君もジムに行ったなんて知らなかった。

BACK INTO ENGLISH

I didn't know you went to the gym either.