The following basically talks about Goku's power level and then ties it in with Gogeta.
With the way power levels increase in DBZ, by the end of the Frieza saga Goku was likely capable of mass destruction on a galactic scale. Which brings me to my next point. Some will call this next point non-canon in the extreme as this is a movie reference. If you wish to do that, that's your choice. I do believe the movies follow the same "rules" as the anime/manga though. There is actually one power level stated to be higher than SS Frieza saga Goku. According to V-Jump magazing, LSS Broly has a power level of 1.4 billion.
Broly destroys the entire South Galaxy in his movie. That's a rather misleading term though, and probably doesn't mean what you think it does. In real life, a galaxy is 50-100 billion planets. There are about 600 sextillion planets in the observable universe. The South Galaxy in DBZ is not a typical real life galaxy, it is one quarter of the universe. So assuming our universes are the same size, Broly destroyed about 150 sextillion planets in one blast. I believe it's been shown that at higher super saiyan forms, possibly as early as FPSS, Goku is stronger than Broly. As a super saiyan 3, Goku is certainly more powerful than Broly, and super saiyan 3s have been shown to be able to break the laws of reality. So it's likely that SS3 Goku could destroy the universe, maybe even reality itself. How fast is Goku though? Well, there's only two good events that you can use to measure speed in DBZ. One is the kamehameha wave from Cooler's Revenge. In about 10 seconds, Goku's kamehameha wave reached from the Earth to the Sun, making it about 50 times faster than the speed of light. Therefore, anyone that can dodge SS1 Goku's kamehameha wave, or even see it, has to be at least 50 times faster than the speed of light. Why is seeing it important? In DBZ, there are many times that the characters move faster than other characters can see. So basically, any villain post SS1 that can keep up with Goku can move at least 50 times faster than lightspeed.
The other case is a little more extreme though. When Vegeta uses The Final Flash against Cell. It's clearly shown that his blast leaves our solar system and explodes on a far away planet/star. The closest planet to our solar system is 10.5 lightyears away. That's 31556926 light seconds. It got there in only 20 seconds, therefore The Final Flash moves at least 1577846.3 times faster than lightspeed. And that was just ascended Super Saiyan Vegeta. Keep in mind perfect Cell saw this attack coming, yet is unable to see SS2 Gohan's movements. And SS3 Goku is obviously far stronger and faster. I don't think Goku would even need to transform to beat Pre Crisis Superman. He could likely kill him with a single finger. To put another nail in the coffin though, let's talk about fighting skills.
Goku is a world champion martial artist that continuously trains to better himself. Many times he has fought evenly with beings of similar or greater power.If Goku's power level increases at the same rate till the end of DBGT as it does till the end of the Frieza saga, as a SS4 Goku would have a PL of roughly 939 Quinoctogintillion. For reference that is a 260 digit number. A PL of 14,600 is required to destroy an earth sized planet. There are about 2 nonillion earths worth of mass in the universe. That means SS4 Goku can destroy the universe about 32 OCTOSEXAGINTILLION TIMES OVER. There's a reason they made Goku a god at the end of GT, and by then ssj4 Goku=ssj4 Vegeta< ssj4 Gogeta.....point proven.