Howard does well enough of portraying this adoration of Books, without overplaying too much, and nicely has just some slight other moments to give Gillom just a bit of variation in personality so he's not purely defined by that, even though that's mostly the point of his character. Although at first the young Gillom is mostly just getting his kicks from his slight interactions with Books, as well as his interaction with anything possibly associated with him. Eventually though Books allows Gillom to go shooting with him, and after they try Howard portrays Gillom as being appropriately cocky after he shoots almost nearly as well as the legendary gunfighter. This leads Books to let Gillom know about the truth behind killing man, which has more to do with will than accuracy. The scene belong unquestionably to Wayne, but Howard does offer some fine support in portraying the way the initial cockiness runs from Gillom face, as he begins to portray a more honest understanding of Gillom towards Books as a man instead of a legend.
Books eventually has Gillom gather the local wannabe gunmen who would not mind to gain the fame from ending the life of the legendary Books. This leads to a final showdown which stands in Books favor it was not for another shooter in the mix, which causes Gillom to step in and live out the moment that he no doubt fantasized in the past. Howard actually is very good in this moment giving the appropriately emotional intensity as Gillom shoots the man, as he portrays that will of conviction to kill in the moment, not for personal glory though but rather something rawer since he's only doing to avenge Books. Howard continues to be very effective as he portrays Gillom sudden realization of what it feels to kill, and shows the complete lack of glory in the moment. I will say much of his performance Howard does not make that much of an impact, although his performance works just fine in the context of what he is given to do. I'll give credit where it is due though since he does give some of the needed power to the pivotal final scenes of the film through his performance.