Who is said to be stronger future trunks or cell saga gohan 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.