And Then There Was Pikkon...

[MC]

Fusion Reborn never happened in regard to the main plot.

All of the Z warriors have became several times stronger than Frieza and some have became several times stronger than Cell (and Majin Buu). A good example of this is how weak Trunks from the alternative time-line was against the androids and Cell, then later on when he goes back to his time-line he was able to defeat the androids and Cell as if they were nothing at all.

Mr. Satan isn't a weakling by normal standards though by the Z warrior standards, yes, he's a weakling.

In Dragon Ball GT, even though Goku lost some of his power due to him being wished into a kid, he was still much stronger than Frieza and Cell at the time. Goku went through a wide-range of strong opponents, so it's only natural that his original over-powering opponents were childs-play compared to him.

 

[Ravendust]

Although it's never stated how strong Goku is at the start of GT (after Uub's graduation), a shared theory is that he's stronger in his normal state than Kid Buu;

-Kid Buu is reincarnated as Uub.
-After Kid Buu's demise, Goku trains for ten years, then fights Uub at the tournament.
-Uub's power increases the more he fights, as Kid Buu's power gets more and more drawn out. Eventually, Goku and Uub are on par with each other (though Uub probably has a lot more hidden power at this point).
-Goku and Uub fly off into the sunset.
-Start of GT.
-Goku and Uub have trained together for ten solid years. It is safe to say that all of Uub's power has been brought out, and that he's even stronger than Kid Buu due to the years of training with Goku. Goku must also be stronger after ten years of training with someone so strong.
-Goku and Uub fight, and are more or less even.

This means by the time GT starts, Goku in his normal form is stronger than Kid Buu