TL;DR: Changes in graphical format: https://imgur.com/a/aiqP9

In ElderThings_Weapons.xml (found in <Steam_Dir>\steamapps\workshop\content\294100\882126182\Defs\ThingDefs_Weapons)

Change line 29
from
<ThingDef ParentName="BaseElderThingWeapon">
to
<ThingDef ParentName="BaseElderThingMakeableGun">

Change line 133
from
<ThingDef Name="BaseElderThingGun" Abstract="True" ParentName="BaseElderThingWeapon">
to
<ThingDef Name="BaseElderThingGun" Abstract="True" ParentName="BaseWeapon">

After line 163 (<unfinishedThingDef>UnfinishedGun</unfinishedThingDef>) add a new line
<researchPrerequisite>ElderThingLaserGun</researchPrerequisite>

After line 164 (</recipeMaker>) add 3 new lines
<weaponTags>
<li>Gun</li>
</weaponTags>

Just to note, this is definitely not the cleanest solution, by the way, but I just can't be♥♥♥♥♥♥♥doing it better.