Gin code respects the XLR_MAX_BLOCK_ID when calling XLogEnsureRecordSpace(). But it appears that the Gist code does not try to limit its block-id consumption to XLR_MAX_BLOCK_ID.
The GIST_MAX_SPLIT_PAGES is hard-coded at 75, but XLogEnsureRecordSpace() would reject a request of more than 32 (XLR_MAX_BLOCK_ID).
The attached patch redefines GIST_MAX_SPLIT_PAGES so that in case of a split, gistplacetopage() now throws an error when the block-ids needed exceed 32.
I have used Min(75, XLR_MAX_BLOCK_ID) as the macro expansion, but I believe it can be set to plain XLR_MAX_BLOCK_ID.