There are different kinds of jamming. It's my understanding that interference jamming is fairly uncommon nowadays. If it's not perfect, it mainly gets you a reduction in SNR and GPS is already way down below the noise floor, so receivers are inherently designed to mitigate the effects even if they aren't explicitly designed for anti-jamming. Even when they work, spectral filters exist. Broadband and spoofing are what Russia typically use. The defenses against those benefit from lots of processing power and large, beamforming arrays.

It's an arms race though.