First you have to be sure the noise is coming from the BB.
As a creak can come from many places and resonate through the frame like a drum: pedals, screws of the ring(s) or the rear hanger not properly tightened, seatpost, stem/bar, loose wheel axles, cassette, rear hub bearings or free wheel parts,...
Ideally check out all this on a home trainer with someone else listening where the noise originate. And try to replace the easy part you think could creak, eg: pedals, rear wheel
Then, or if you are certain it is the BB.
Check the smoothness of the non drive side bearings, it is the one that generally dies first with gxp.
If not smooth, replace bearings. Either with a good brand SKF, NTM,... that will last a little longer, or buy them by the dozen on aliexpress and consider it as a disposable part.
The wavy washer is there to take any slack and preload the non drive side bearings that takes all lateral load.