In Dragon Ball GT Kid Goku goes super saiyan 4. This is after Battle of Gods and Resurrection F so when he needs to fight why wont he go god form he needs more power and there is no time to how long he can be in it.
why does goku not go super saiyan god when he versus omega at full power with all the balls
If you want to consider GT a sequel to Super, then you can try to rationalize this by looking at Gohan: He lost a great deal of his power by not training. I think he may have even lost the ability to turn into a Super Saiyan. The same could happen to Vegeta and Goku between Super and GT. Nevertheless, I expect more contradictions will continue to crop up between Super and GT, making it difficult to be an apologist for such an unpopular stance.– Shaymin GratitudeMay 23, 2017 at 22:10
First off because Dragonball GT was created before Super or the 2 movies which the first 2 seasons are based from was even created. as such Super Saiyan God and the levels after that shown in Super weren't even conceived when Dragonball GT was written. this along with other contradictions in this answer by Torisuda does make Dragonball GT incompatible with Super.
also to quote the Wikia
In December 2014, FUNimation voice actor and voice director Christopher Sabat said that GT is "not even canon anymore", likely meaning that the release of Dragon Ball Z: Battle of Gods superseded GT as official content, in Sabat's opinion. Notably, Sabat's statement is the only recorded usage of the word "canon" by anyone involved in the production of any Dragon Ball media.
Source: Dragon Ball GT > Overview > Toriyama's involvement and canon debate (last Paragraph)
Added to that many fans do not consider GT cannon as there is no Manga written by Akira Toriyama and also comparing the Wikipedia Pages of GT and Super Akira Toriyama isn't created to have written GT unlike Super, as such Super Saaiyan Levels in GT weren't created by Akira.
however if you still want to think of GT being in any way canon, considering the multiple universes shown in Super you can suspend belief and think of GT being in Universe GT where the events of Super never occurred and (until otherwise indicated) no one can travel to Universe GT