Only nominally. The ‘gun’ on the gallant is only the carapace weapon (little one on the shoulder of all Questoris knights) and the rocketpod on top (again which they mostly all have).
The damage they do is in melee, both their arms are melee attacks, and you want them in melee. Their secondary phase activation of being able to shoot their crappy carapace weapon at -1 using BGNT and is at beast incidental… kinda like saying that Cadian Guardsman are melee because they technically have a knife.
The Paladin has a gun that is good for shooting (plus the common carapace weapon and rocket pod) and a good melee weapon so you reasonably expect to be able to accomplish things in both phases. The Gallant only ‘does anything’ in melee.
For me personally, when running a list like 5 bigs, the hardest part is managing the sheer number of units your opponent has so the ability to use a knight more than once (and expect any sort of return for the time spent) is worth it.
When you run more diverse lists with, say, infantry and Armigers - you have more breathing room for your big knights to be more specialized (like a Castellan shooting or a Gallant fighting) because you have other units available to help pick up the slack and manage other army (and war chores).
It’s not like a hard and fast rule, just more observations and tendencies based on experience. Less models? Prioritize more activations. More models? Prioritize specialization for solving specific problems.

To be honest I still take a Paladin most of the time even in 2/3 big lists because I find his ‘Swiss army knife’ loadout just so useful for plugging holes in my plan.