Granblue fantasy

You can, but only if you sparkle during the Flash Gala.

Yes and no. There are only about 10 changeable elements that can be sparkled, and Kosh is right. New characters (when available) and themed items for this gala are guaranteed to be available. When I sparkled Zoe, I was just lucky that there was a summer theme at the time (Summer Jeanne was also available, for example. And Beatrice, I think. Note that not all golden summer characters were available).
That is, in theory, it is not necessarily possible to sparkle a specific character in a particular gala. Even in premium and flash galas.
In this regard, paid sets with a ticket to choose a character are more reliable. There are more of these characters there. But if you don't want to pay, you need to pay close attention to when you sparkle. Sooner or later, what you need should appear.
There should be a story from our guild about Kit (Whale_kid), who reached high levels, donating whenever possible. He is either rich or has already sold his apartment, but he had about 300 golden characters (possibly all of them in the game) and he said about weapon stones: "Why are you running out of them? That's strange. Any weapon in the game gives 100 of them when reduced." This suggests that he has never farmed, only drooled.