Short answer: No gravel!

If you dig a hole in your garden in the wintertime, does it fill with water and refuse to drain? Does water lie on top of the ground for days at a time? If not, your drainage is fine, and you don't have to do anything special. When your raspberry gets established, you will find out how meaningless all these special techniques are, as it attempts to take over your yard.

raised bed = plenty of drainage

mounding = plenty of drainage

gravel under the rootball = plenty of drainage

Any combination of two or more from above list = what, are you trying to grow Saguaro cactus?

Much of the Puget Sound area has glacial gravelly till soil which has all the water retention of aquarium gravel. What are officially classified in soil surveys as 'excessively drained' soils. Unless you're in a pocket of clay or peat, and sometimes even then, you've got enough drainage for all but the most demanding plants from dry climates. Raspberries don't qualify for that group. They're a weed here.

If water doesn't stand for long periods during rainstorms then you've got adequate drainage. Last winter doesn't count - after a couple of weeks of nonstop rain we had standing water in places water never stands normally, and then it kept raining. and raining. and raining... for 2 months. Nothing could drain, the soil was totally saturated long before it stopped raining.

Where are you getting your gardening info from? sources intended for a national audience are worthless on the west coast. Everything is different here. Soil, precipitation, climate, everything. Stick to sources written specifically for the west coast, or the British Isles. They have a similar climate at least even if the soils are different.

So plant your raspberries in the raised bed and call it good. It will be plenty good enough.