No, Goku is not unbeatable. As a matter of fact, it is one of the running themes of the Dragonball Series that Goku is often defeated, is either forced to retreat or killed, and while retreating or "dead" trains his abilities up to new levels. The rest of the Z-fighters often have to hold the fort while he trains.
Son Goku's power level varies widely across the length of the Dragonball, Dragonball Z and Dragonball GT sagas. What is consistent about Goku is his natural talent for combat, his zeal and dedication to training and personal improvement.
Goku starts Dragonball with loads of natural talent. Even as a small child after his arrival on Earth he starts being trained by first by Grandpa Gohan and then later Master Roshi. Goku is quite formidable at the age of about eleven or twelve. His mimicry abilities were so great, he learns the Kamehameha wave attack just by watching Master Roshi do it. Once.
By the time he is a teen, he is arguably one of the strongest martial artists on Earth. And with the addition of the Kamehameha wave, (slow as the attack might be back then) it still gave him a major advantage.
It wasn't until after his death at the hands of his brother Raditz, did Goku realize how much stronger he needed to be. His time training with King Kai expanded his mind about his potential and he dedicates himself to being the best he can be.
Goku exemplifies the mind of the martial student. He embraces both his training and new abilities learned. He is flexible and focused. But most of all he is hard-working and willing to put in the time and effort it takes to become the best. Throughout the DragonBall Series, Goku continued to put forth the greatest amount of time training his abilities under diverse Otherworld masters.
Contrast this with Prince Vegeta, who while also having significant natural talent, was at least at first, less willing to train his abilities, expecting his talent to carry him to ever greater levels of power. Since Saiyans powers increase when they fight powerful enemies, Vegeta expected his battles to improve his powers more than training did. Vegeta was also more inclined to use magic or technology to alter/increase his powers in his early career.
As far as Gohan is concerned, the series mentioned a couple of times that he had the potential to be far more powerful than Goku and that under the right conditions, his training at an extremely young age with Piccolo increased his powers to a level even greater than Goku (for his age).
- During the Cell Saga, it is Gohan's ability to release his power which destroys the deadly menace of Cell, one of the Z fighters greatest enemies.
- As Gohan grew older his powers seemed to taper off, likely because he had interests other than combat, he seemed to peak out at Super Saiyan 2 while Goku continued to develop and improve. This did not mean he was not powerful, it simply meant he did not continue to develop his Saiyan heritage. His mother may have had something to do with that as well, since she encouraged him to continue his schoolwork.
Goku's power level increases after his death at the hands of Raditz and his training with King Kai, his new power level is over 9000(!), and continues to rise from that moment on, arguably becoming one of the most powerful beings in the Dragonball Universe.