I mean i know that gohan beat cell and all but in dragon ball super ssj rage trunks beat merged zamasu which technically is considered two gods(goku black and zamasu).
Cell was supposed to be the perfect being in terms of abilities and power. At the time he was the strongest villian. The god of destruction and the universe hierachy was introduced in DBS with Zen, Beerus, Whis etc. So therefor it is pretty difficult to tell if Cell would've had a chance against Beerus; a god of destruction.
It was said in DBS that Zamasu killed ALL the gods of destruction in the future timeline. Cell was defeated by Saiyans, from which they are in DBS still weaker than a god of destruction. Keep in mind that during DBZ they were significantly weaker (apart from Gohan since he was still training a lot), but he was never close to the strength of Goku; not to mention Beerus. Beerus was defeated by Zamasu in the future timeline which means that cell is weaker than Beerus and therefor weaker than Zamasu.
Zamasu being beaten by someone means that that certain someone is stronger than all that Zamasu was able to beat (going by logic). But we know from experience between son and father; Vegeta and future Trunks, that future Trunks is still way too weak for that level of strength.
From what I recall during the arc of Zamasu, Trunks didn't beat him. He simply survived a brawl so now and then. As shown in the battles against Goku and Vegeta compared to that with Trunks he was never serious towards Trunks, so Trunks never really beat him. If he did he wouldn't go through so much sacrifices and trouble in order to get Goku and Vegeta.
For all I remember, Zamasu was so far the most powerful villian. Goku had to call Zen-Oh in order for Zamasu to be eradicated. Hence the reason why there are 2 Zen-Oh's in the current timeline.
So all in short, Zamasu was only ever defeated and properly beaten by Zamasu. Therefor it is unknown how "actually" powerful DBS future Trunks is compared to DBZ Gohan from Cell-saga. Due to major differences in plots and storyline it is also hard to guess. There is lack of information about Trunk's full strength in effect in DBS.
Dont quote me on this, but I'd call DBS future Trunks stronger than DBZ Gohan if it was based on opinion. Let me explain why. First of all, age difference essentially means difference in experience for combat. Trunks has been fighting for several years in the timeline where there is no Goku meaning he had a lot more time to train. Gohan was still young in DBZ and therefor lacked the years of experience Trunks has. As for Saiyan states during cell-saga Gohan attained ssj2. His ultimate saiyan form is basically ssj2 with more power at the cost of speed. So as for saiyans both are equally strong. But due to experience I'd say that Trunks would win. As shown in the most recent dbs episode (114), Goku ssj1 was able to withstand ssj2 due to his fighting experiences and Trunks is no fool while gohan still was at that age, that's why my vote goes to Trunks.
Well, your question isn't very accurate. At the end of the cell saga, SSJ2 Teen Gohan was a lot stronger than future trunks. Also, it is fair to assume that by the end of the Buu saga in Dragon Ball Z, Gohan was superior to future Trunks. We know this because Future Trunk's mention's how he turned SSJ2 to fight off Dabura while Gohan's Mystic form had made him a lot stronger.
Now with respect to Dragon Ball Super. Future Trunks had mastered SSJ2 and he was strong enough to push back Goku SSJ2, however, 1 hit from SSJ3 Goku was enough to knock him out. Gohan, on the other hand, had slacked off on training and was a lot weaker. Future Trunks was stronger than Gohan even then, we can confirm this based on an event which happens later which I will mention. Trunks SSJ Rage transformation is an extremely powerful transformation which puts him even above SSJ3 Goku but not as strong as SSJB Goku. It's a transformation which would more or less make him even stronger than Super Saiyan God because he was able to hold off Black who was making sport of 2 Super Saiyan Blues. Also, he was able to hold his own against Merged Zamasu(Despite the spirit sword, he still did manage to hold his own).
Now before the tournament of power, we see Gohan train with Picollo. We see Picollo being stronger than SSJ2 Gohan at that time. So it is fair to assume Trunks was easily superior to Gohan at the time he arrived. After Gohan regained his Mystic form and trained further, you could say even he crossed the Super Saiyan God level but isn't as strong as Super Saiyan Blue Goku or Vegeta.
This leaves us with a question as to whether current Mystic Gohan or Future Trunks SSJ Rage is stronger. Without a doubt, it would be Future Trunks' SSJ Rage. However, we haven't seen Gohan fight at full power. I'm pretty sure once we see him fight someone strong in the tournament of power, we can clearly determine whether he surpassed Trunk's in terms of power or not.
Cell Saga Gohan is stronger than Future Trunks during the Cell games. He was a transformation level over him (SSJ2 vs SSJ1)
Future Trunks in Dragon Ball Super was more or less in the same level than Cell Saga Gohan (SSJ2 vs SSJ2) but later he got a new transformation (Super Saiyan Rage) which is equal or over than Super Saiyan Blue, which is 3 transformations ahead than SSJ2 (SSJ3, SSG, and SSB) . When Trunks became Super Saiyan Rage, Yajirobe states that he might be equal to Black's Super Saiyan Rosé, which by the time was beating badly Goku and Vegeta's Super Saiyan Blue. Later Vegeta trains and Goku and Vegeta get zenkai boosts so they get able to overpass Black's Super Saiyan Rose, so their versions of SSB get stronger than Super Saiyan Rosé (which is equal or stronger than Super Saiyan Rage). But later Trunks gets more furious and he's able to almost hold his own against a beam struggle with merged Zamasu (which again became stronger than Goku's and Vegeta's Super Saiyan Blue) later Goku's Super Saiyan Blue is able to hold his own against a beam struggle against merged Zamasu and finally it's Trunks who defeats merged Zamasu (though at some point he gets genki dama energy from all people) so it looks like Super Saiyan Rage is surpassing and being surpassed by Super Saiyan Blue in cycles all the time, and SSB is a transformation 3 stages above SSJ2, so Super Saiyan Rage is clearly above Cell saga Gohan.