Does any one know if it's possible to describe formula in saved search, like this? {createdfrom.custbody_gpn_saleschannel.custrecord_gpn_channelid}

you can only do a join to an adjacent record in saved searches. so {createdfrom.custbody_gpn_saleschannel} will work but not your example. You could possibly refactor your search to look at the applying transaction (or applied to) and then make the jump to the channelid

Thanks!