make a file called PlanetModule_WeaponDefense.entity
(word pad works fine for this)
I assume u know where the folder is for this... since u said u tried modding before.
some numbers are different here on the item but i highlighted the number below, for the range. Its good enough to cover that side of the planet

TXT
entityType "PlanetModuleWeaponDefense"
minZoomDistanceMult 2.000000
ability:0 ""
ability:1 ""
ability:2 ""
ability:3 ""
MaxAntiMatter 0.000000
AntiMatterRestoreRate 0.000000
numRandomDebrisLarge 3
numRandomDebrisSmall 19
numSpecificDebris 0
armorType "VeryHeavy"
hudIcon "HUDICON_PLANETMODULE_TECHORBITALGAUSSDEFENSE"
smallHudIcon "HUDICONSMALL_PLANETMODULE_TECHORBITALGAUSSDEFENSE"
infoCardIcon "INFOCARDICON_PLANETMODULE_TECHORBITALGAUSSDEFENSE"
planetUpgradeSlotType "Tactical"
planetUpgradeSlotCount 4.0000
basePrice
credits 250.000000
metal 75.000000
crystal 35.000000
BuildTime 55.000000
NameStringID "IDS_PLANETMODULE_TECHORBITALGAUSSDEFENSE_NAME"
DescriptionStringID "IDS_PLANETMODULE_TECHORBITALGAUSSDEFENSE_DESCRIPTION"
planetModuleRoleType "WEAPONDEFENSE"
statCountType "ModulePointDefense"
Prerequisites
NumResearchPrerequisites 0
MaxHullPoints 3000.000000
HullPointRestoreRate 2.000000
BaseArmorPoints 12.000000
MaxShieldPoints 0.000000
ShieldPointRestoreRate 0.000000
maxShieldMitigation 0.0
ExperiencePointsForDestroying 50.000000
angularSpeed 19.999992
rotateSoundName ""
resourceExtractionType "Invalid"
cultureSpreadRate 0.000000
rotateToFacePlanet FALSE
rotateConstantly FALSE
isAffectedBySimilarModuleCostResearch FALSE
ShieldMeshName ""
NumSoundsFor:ONATTACKORDERISSUED 0
NumSoundsFor:ONCREATION 0
NumSoundsFor:ONGENERALORDERISSUED 0
NumSoundsFor:ONSELECTED 1
SoundID "PLANETMODULE_TECHORBITALGAUSSDEFENSE_ONSELECTION_0"
NumSoundsFor:ONSTARTPHASEJUMP 0
MeshName "PlanetModule_TechOrbitalGaussDefense"
ExplosionName "PlanetModuleSmall"
mainViewIcon "MAINVIEWICON_PLANETMODULE_TECHORBITALGAUSSDEFENSE"
picture "PLANETMODULEPICTURE_TECHORBITALGAUSSDEFENSE"
minShadow 0.000000
maxShadow 0.600000
formationRank 3
defaultAutoAttackRange "None"
defaultAutoAttackOn TRUE
prefersToFocusFire TRUE
NumWeapons 1
Weapon
WeaponType "Projectile"
damageEnums
AttackType "ANTIMEDIUM"
DamageAffectType "AFFECTS_SHIELDS_AND_HULL"
DamageApplyType "BACKLOADED"
DamageType "PHYSICAL"
WeaponClassType "GAUSS"
DamagePerBank:FRONT 600.000000
DamagePerBank:BACK 0.000000
DamagePerBank:LEFT 0.000000
DamagePerBank:RIGHT 0.000000
Range 8500.000000
PreBuffCooldownTime 6.000000
CanFireAtFighter FALSE
SynchronizedTargeting TRUE
PointStaggerDelay 0.000000
TravelSpeed 4000.000000
Duration 0.000000
WeaponEffects
weaponType "Projectile"
burstCount 1
burstDelay 0.000000
muzzleEffectName "Weapon_Tech_OrbitalDefense_Muzzle"
muzzleSoundMinRespawnTime 0.100000
muzzleSounds
soundCount 1
sound "WEAPON_TECH_GAUSSCANNON_MUZZLE"
hitEffectName "Weapon_Tech_OrbitalDefense_Hit"
hitHullEffectSounds
soundCount 1
sound "WEAPONIMPACT_PHYSICALHEAVY_HITHULL"
hitShieldsEffectSounds
soundCount 1
sound "WEAPONIMPACT_GENERICHEAVY_HITSHIELDS"
projectileTravelEffectName "Weapon_Tech_OrbitalDefense_Bullet"
m_weaponIndexForRange 0

You dont need to make any new files if your willing to change the main game info folder.Even if you dont want to do that you could just make a new folder in the mod part and copy the gameinfo folder into it.Then you just go into the gameinfo folder and look for PLANETMODULE_TECHORBITALGAUSSDEFENSE.Open it with notepad and just find the range and change it to whatever you want.Then enable it in the main game.

You could also grab the beta mod pack which has all the 1.02 files in their text format, so you don't need to edit anyone else's mods (which probably would be converted back to binary anyway )

Beta pack

Well, I've followed the directions, and the mod shows up in the game mod menu, but it seems that none of the changes I make have any effect. Range, damage, and hit points all stay the same ingame even after enabling the mod. What the heck am I doing wrong?

Make sure you save the changes you make.Maybe you didnt do that.

Or tired. I wasn't paying attention and accidentally deleted a part of "StringInfo" once. The result was quite comical

Haha.. okay. I was deleting a string from the English.str file, and when I was selecting I accidentally clipped the 'S' in StringInfo on the following line and didn't notice, so it ended up reading 'tringInfo' when I saved.

Of course, the engine got quite confused at this, and decided not to load any subsequent strings - the result (since the typo was pretty high on top), all the main menu buttons up until game load read "STRING NOT FOUND". And after the game load, it just minidumped