Why is Trunks not as powerful as Gohan when they are both human-Saiyan hybrids?
When you say Trunks, I'm assuming you are talking about Future Trunks. However, the answer really boils down to training. In the Android Saga, Future Trunks mentions that Androids 18 and 19 are much stronger than the ones in his timeline. Future Trunks never got the training of Goku, everyone was already dead, and training was virtually impossible, though he did get a little in. Potential for great strength, then achieving great strength are two different things. Had Trunks went through gravity training like Gohan and the Saiyans, he WOULD have been MUCH stronger. As it is, present Trunks only does gravity training in the Hyperbolic Time Chamber, and only for a little bit. 

Another point to see is that present Trunks (kid version) is stronger than Frieza when Goku first fought him. His Super Saiyan form is already equal to if not stronger than Goku's power against Frieza in that fight, so he IS super strong. Everyone else is just that much stronger. This is the reason he never caught up to Gohan/Goku/Vegeta—they already achieved his strength (much later on), and KEPT GOING. If he had grown up fighting/training only, he would be one of the top fighters.