How do seagulls deal with high salt in their diet?

Yes they have special glands which filter out the salt.

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You can't "filter" the salt out, but you can remove it by osmosis, and all animals - including us - absorb most of the water they drink by osmosis into the blood-vessels within the intestinal walls. An animal that can drink brine does the same, but its osmotic process has developed to transfer the water but leave the salt behind.

It does not use "special glands". Anyway, a gland produces enzymes, hormones other metabolic chemicals; it is not a filter.