The yellow/white gunk is water from one of 2 things or both.
A: a defect in the water to oil interface (1: head gasket, 2: crack in head, 3: crack in block )
B: moisture from the air the engine breaths condensing inside the engine.
So first, what is your normal drive cycle, or how long / how far do you normally drive. With the normaly quick engine warm up time, any thing longer than 2 miles should be long enough (summer time mid-US). Cooler climates will take a bit longer. Now many very short hops and never getting the engine up to temp for long enough allows the moisture to collect and condense into a “yellow/white gunk”
To test for water leak one can get a good pressure gauge and pressurize the cooling system and see how well the system holds pressure.
For a short term repair of a water leak, one can try one of the many “coolant leak stop” products on the market.