Manwe, Pallando wasn't asking about the form in which Gandalf returned - he asked about the form he took on while he was gone (the time he spent in Aman between the Battle of the Peak and his return to Middle-earth).
The answer is not given to us - did he appear as the old man we see in Middle-earth, or did he appear as he had earlier in Aman as Olórin? This isn't covered by Tolkien. I do not have any reason to believe one over the other except for that the 'old man' appearance was made specifically for him to travel to Middle-earth without drawing a lot of (negative) attention. It seems only natural that this incarnation was not how he appeared in Aman.
Additionally, we're not even sure how such (Ainur) spirits were presented in Aman.
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...take counsel with thyself, and remember who and what thou art.
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