YOU SAID:

I am planning to go to the gym tomorrow morning.

INTO JAPANESE

明日の朝ジムに行く予定です。

BACK INTO ENGLISH

I am going to the gym tomorrow morning.

INTO JAPANESE

私は明日の朝ジムに行きます。

BACK INTO ENGLISH

I will go to the gym the next morning.

INTO JAPANESE

私は翌朝ジムに行きます。

BACK INTO ENGLISH

I will go to the gym the next morning.