I have another hypothesis about index bloat cause. AFAIK, vaccum procedure on GiST don't have any storage utilization guarantee. For example, if only one live item is in some page, then only one item will be left in this page. I.e. there is no index reroganization during vacuum. If there wouldn't be many inserts into such pages in future then they will be stay bloat.
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With best regards,
Alexander Korotkov.