Yes, you can choose Animations: All
:
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You can't choose individual animations though, so you'd need to delete all the animations except the ones you want to combine into one GIF. Save your project first!
The animations are played one after the other, in the order they appear in the tree. You'd have to rename them to get a different order.
Another option could be to export individual GIFs, then combine them with another tool, such as ImageMagick.