In the first edition compare bar 10 in the first Promenade with bar 10 in Promenade V which appears after "Samuel" Goldenberg und "Schmuÿle". This time bar 10 has G flat.
So the missing flat the first time could be a typo.
Now take a look at this Wiki article:
Quote from that article:
As with most of Mussorgsky's works, Pictures at an Exhibition has a complicated publication history. Although composed very rapidly, during June 1874, the work did not appear in print until 1886, five years after the composer's death, when an edition by the composer's friend and colleague Nikolai Rimsky-Korsakov was published. This edition, however, was not a completely accurate representation of Mussorgsky's score but presented a revised text that contained a number of errors and misreadings.
Only in 1931, marking the 50th anniversary of the composer's death, was Pictures at an Exhibition published in a scholarly edition in agreement with his manuscript, to be included in Volume 8 of Pavel Lamm's M. P. Mussorgsky: Complete Collected Works (1939).
In 1940, the Italian composer Luigi Dallapiccola published an important critical edition of Mussorgsky's work with extensive commentary.
Mussorgsky's hand-written manuscript was published in facsimile in 1975.
Based on that I think we can conclude that the G flat printed in later editions is correct.