The first time when we saw a character with a halo in Dragon Ball we have to look back to Gokus childhood where he met his dead Grandpa Gohan at Babas place. People who die in the universe of Dragon Ball are in special cases allowed to get a recreation of their bodies in the afterlife.
The only difference is that dead people with their real body have halos placed above their heads. A few years later Goku made the same experience after a sacrifice to beat his brother Raditz. In some anime fillers later we also see dead evil people like the Ginyu Force having halos.
But also Dragon Ball Z have inconsistencies concerning dead villains in Hell. People coming to hell aren't supposed to have a body. During the fight against Majin Buu Vegeta asked Piccolo what will happen to him when he would die in order to kill the demon. The Namekian replies that he will lost his body because he has killed too many innocent people lately. But why could Frieza or Cell keep their real body?
Let's get to that later. But despite we saw so many villains in hell got halos, why did they lost them in the Super 17 Saga in Dragon Ball GT? It's not to compare with the movie Fusion Reborn, in which the dead people become alive again. Why even people appeared which haven't died on Earth is still another question.
During Dragon Ball GT everyone from Hell could easily 'jump' to Earth through a hole made by two Android #17 and nobody of them had a halo before. Interestingly, after being killed again waiting in the queue at Enma-Daios place enemies like Puipui still have halos above theirs heads. They lost them probably again on their way to hell.
Piccolo was seen losing his halo after destroying parts of the paradise which means he was about to go to Hell - even if it looked similar with the situation where he was revived at North Kais planet in the Frieza Saga before getting transported to Namek.
But people in Hell still remain to be indestructible. Even cutting Frieza in half didn't help to defeat him, because his body reshaped - same as in Dragon Ball Z with characters having halos in similar situations.
To distinguish people in Hell without halos between dead or alive the body temperature makes the difference: The temperature of dead people is much colder than from alive ones. That's why the ice machine worked with Frieza and Cell, but not with Goku.
Anyway, we can say that the makers of Dragon Ball GT obviously intended the change to remove the halos from the people living in Hell. It's a shame it wasn't explained why.
Maybe they didn't want villains to have halos because of the Super 17 plot? A change made by Enma-Daio? Nothing makes really sense when such a big thing isn't explained at all. Dragon Ball GT has its own rules and this thing is absolutely a case in point.
