Hrm... rather than editing the script files, which would impact saves, perhaps do it the other way around and have the gallery file fetch info itself from loaded save file (and its history), then check to see if corresponding info is in the gallery yet or not, and if not update that spot. I don't know how far can be searched, but if certain scenes use a specific word that is not used elsewhere in the VN, could search for that exact word in place of a variable. An example would be "Artificer" to correlate with the CG right before the fight/struggle, while that word may get reused, you would have had to have seen that CG first anyway to get there. The harder part is that is core or non missable scene, so the same may not be able to be said for optional scenes.
Worst case is if you have to put variables into the script files (and saves stop working), you will need a chapter or part select feature to help viewers get back to where they were in haste time. In this case I would seek to ask for coding help from how Distant Travels did its Chapter Select, where it skips right over to each decision point there was along the way (thus filling in the variables), with an additional "Start Here" (at this decision point) option.
Always options on how to give options, lots to think about, but don't overthink it. Stop staring into space Melmond! XD