So the sink never used to gurgle but now it does and the gurgling is gradually getting worse.
So it can't possibly be anything to do with the pipe layout - unless the pipes are moving about and gradually making the gurgling get worse - so it's probably something to do with an obstruction in the pipes building up over time. Shaving foam is my guess - gradually accumulating and slowly blocking a pipe - air wanting to come upwards is fighting water wanting to go downwards - the gurgle is the struggle between the two.
Swap to shaving oil (olive oil is fine) and get some caustic soda - wear rubber gloves - mix a small amount up in the sink and pull the plug - do it several times and listen carefully for a change in the gurgling sound. Take care not to use too much caustic soda - if you do it may form a solid plug and block everything - keep the water moving. Keep the solution away from paint skin and eyes - do not attempt to shave with it.
Good luck...
Peccavi
May 2009