Mind the fact that Goku is nearly double the age and double the experience in martial arts and working out compared to Garou, he clearly has an understanding of the body, human, Saiyan or otherwise (see the numerous alien foes he defeats for reference).
Goku excelled at fighting before Garou even walked into a dojo, and was able to learn even difficult things in record time. Often for story reasons he picks it up instantly or it takes a while (Kamehameha vs Instant Transmission, a few seconds to perform identical to a master compare to months to become adept). Garou is also a near master at adapting and picking up things, but his actions clarify he is willing to drop how he fights if he thinks he can win (see him vs Saitama). He just goes to the deep end with a clear drop in skill, a literal death sentence for a fighter like Goku who capitalizes off his enemies getting sloppy (Piccolo, Freeza, Vegeta, etc)