I wanted to ask, if one performs ghusl of janabah and makes the intention to wash his body in parts ie, he first makes the intention to wash his head face and neck, then when washed, continues to make the intention to wash the right side of the body. If he does this and completes his ghusl and when finished doubts whether he made the *intention* to wash his *left* side AFTER completing his ghusl, is he required to perform it again despite washing his left side completely as well?

If you make the intention for the ghusl in the beginning and then wash head and neck followed by the right side then the left side it’s valid.
Ignore your doubts especially when you are done from the act