How do Advaitins interpret Bhagavad Gita 7.24?
However, this seems that the forms of God are real.
No, it doesn't mean that.
...was impersonal before and have now assumed this personality...
says that Brahman wasn't impersonal only before, he is still impersonal i.e the assumption of personality is wrong.
Actually the higher nature of Brahman is imperishable (From Kena Upanishad 2.3 to 2.8 Brahman is not the subject of/for mind, speech, eye etc. & from Taittariya Upanishad (2.4.1 and 2.9.1) यतो वाचो निवर्तन्ते । अप्राप्य मनसा सह ।)
Anything is called Sat (सत्) only if it exist (and remains constant) in all the three periods of time (past,current & future) i.e त्रिकालाबाध सत्य. In Advaitin saying "Brahma Satyam" it also means that Brahman is not subjected to change or transformations. (Paramarthika level, GK 2.32) So, manifestation would be unreal.
I suggest you to read verse 8.20:
यः स सर्वेषु भूतेषु नश्यत्सु न विनश्यति।।8.20।।
8.20 But distinct from that Unmanifested is the other eternal unmainfest Reality, who does not get destroyed when all beings get destroyed.
Note: “The question: How do Advaitins interpret Bhagavad Gita 7.24?” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.