Zombie Survival now sports the following custom hooks and stuff.
If you're adding new game features then you would probably want to use hooks instead of overwriting files that may be updated.
You're a big boy, you know how to make hooks and use autorun so I won't go in to that. Just use them as if they were any other hook.
Returning anything besides nil in a hook will override the gamemode's stuff.

If you're going to be calling gamemode functions then use gamemode.Call so hooks are called and everything works properly.
gamemode.Call("EndRound", TEAM_HUMAN) instead of GAMEMODE:EndRound(TEAM_HUMAN)
MOST OF THE TIME THERE IS A HOOK CALLED AFTER THE FIRST ONE CALLED PostHookName

=SHARED=

Hook. Can a player purchase something from a crate? Return false to explicitly not allow, true to explicitly allow.
GM:PlayerCanPurchase(Player ply)

Hook. Can a player checkout their worth loadout?
GM:PlayerCanCheckout(Player ply)

Hook. Called when a weapon is succesfully deployed. Used for setting player walk speeds.
GM:WeaponDeployed(Player ply, Weapon wep)

Hook. Can a player be healed?
GM:PlayerCanBeHealed(Player pl)

Hook. Is a class unlocked?
GM:IsClassUnlocked(Integer class)

=SERVER SIDE=

Hook. Called when the game wants to turn waves on or off.
GM:SetWaveActive(Bool active)

Hook. Called when the game wants to set the time that a wave will start.
GM:SetWaveStart(Float time)

Hook. Called when the game wants to set the time that a wave will end.
GM:SetWaveEnd(Float time)

Hook. Called when the game wants to set the wave to something.
GM:SetWave(Integer wave)

Hook and function. End a round with a team as the winner. Bit of a complication here: zombies don't actually win ZS. The point is to survive so if you die you must redeem to have a shot at winning.
GM:EndRound(Integer team)

Hook. Called when a player repairs an object. Usually a nail.
GM:PlayerRepairedObject(Player ply, Entity object, Float repaired, Weapon wep)

Hook. Called when a nail is created. ent1 and ent2 are the two entities. ent2 may be the world.
GM:OnNailCreated(Entity ent1, Entity ent2, Entity nail)

Hook. Called when someone heals a team member (NOT THEMSELF).
GM:PlayerHealedTeamMember(Player healer, Player target, Float health, Weapon wep)

Hook. Called when there is one human left.
GM:LastHuman(Player pl)

Hook. Called when a zombie kills the last human.
GM:LastBite(Player victim, Player attacker)

Hook and function. Gives a player their default equipment or a random loadout if they have none.
GM:GiveDefaultOrRandomEquipment(Player pl)

Hook and function. Gives a player random equipment without checking their favorites.
GM:GiveRandomEquipment(Player pl)

Hook and function. Loads the next map. HERE IS WHERE YOU PUT YOUR MAP VOTING AND STUFF. RETURN TRUE TO OVERRIDE.
GM:LoadNextMap()

Hook. Called when a player has told the server that GM:HookGetLocal (client-side hook below) has been called. DO NOT TRUST THE CLIENT ON THIS, SHOULD ONLY BE USED FOR COSMETICS AND SENDING NETWORKED STUFF.
GM:PlayerReady(Player pl)

Function. Recalculates if a prop should be frozen or not due to being nailed and such. Use true as a second argument to check all props that have a constraint to this one as well.
GM:EvaluatePropFreeze(Entity ent, Bool usehierarchy)

Hook. Human kills a zombie.
GM:HumanKilledZombie(Player victim, Player attacker, DamageInfo dmginfo, Bool headshot, Bool wassuicide)

Hook. Zombie kills a human.
GM:HumanKilledZombie(Player victim, Player attacker, DamageInfo dmginfo, Bool headshot, Bool wassuicide)

SELF-EXPLANATORY HOOKS AND FUNCTIONS
GM:LoadMapEditorFile()
GM:SetupSpawnPoints()
GM:RemoveUnusedEntities()
GM:ReplaceMapWeapons()
GM:ReplaceMapAmmo()
GM:ReplaceMapBatteries()
GM:CreateZombieGas()
GM:SetupProps()
GM:DoHonorableMentions()
GM:PostDoHonorableMentions(filter)
GM:PostEndRound(winner)
GM:CalculateInfliction()
GM:RemoveDuplicateAmmo(pl)
GM:ShouldAntiGrief(ent, attacker, dmginfo, health)
GM:PostHumanKilledZombie(ply, attacker, dmginfo, assister, assistfrags, headshot)
GM:PostZombieKilledHuman(ply, attacker, dmginfo, headshot, wassuicide)
GM:CanDamageNail(nail, attacker, inflictor, damage, dmginfo)
GM:OnNailDamaged(nail, attacker, inflictor, damage, dmginfo)
GM:OnNailDestroyed(nail, attacker, inflictor, damage, dmginfo)
GM:OnNailRemoved(nail, ent1, ent2, remover)
GM:OnPlayerChangedTeam(ply, oldteam, newteam)
GM:PlayerPointsAdded(ply, points)
GM:PlayerRedeemed(ply)
GM:WaveStateChanged(active)

=CLIENT SIDE=

Function. Adds a death notice.
GM:AddDeathNotice(String attackername, Integer attackerteam, String inflictor, String victimname, Integer victimteam, Bool headshot)

Called when the player sees LocalPlayer() as a valid entity and MySelf is available to be called.
GM:HookGetLocal(Player pl)

Hook. Called when the player dies as a human. attackername is the player's name. Note, this is only called when killed by another player and while a human.
GM:LocalPlayerDied(String attackername)

Function. Adds a floating score number in the world.
GM:FloatingScore(Entity ent, [String type, Integer score, Integer flags])

Hook. Called when the player gets profits from a crate they own.
GM:ReceivedCommission(Entity crate, Player buyer, Integer points)

Hook. Called when the player heals another player.
GM:HealedOtherPlayer(Player other, Integer points)

Hook. Called when the player repairs an object.
GM:RepairedObject(Entity object, Integer pointsreceived)

Function. Suppresses the Arsenal Upgrade message for provided number of seconds.
GM:SuppressArsenalUpgrades(Float time)

Function. Add an honorable mention to the end board.
GM:AddHonorableMention(Player ply, Integer mention, Integer data)

Hook. Called when opening the human context menu.
GM:HumanMenu()

Hook and function. Toggles zombie dark to light vision.
GM:ToggleZombieVision()

Hook. Called when you click a person's name in the scoreboard.
GM:ClickedPlayerButton(Player ply, Panel panel)

Hook. Called when you click a person's name in the honorable mentions.
GM:ClickedEndBoardPlayer(Player ply, Panel panel)

Hook. Called when building the help menu. Use this to add new sheets.
GM:BuildHelpMenu(Panel helpwindow, Panel propertysheet)

Hook. Called when building the options menu. Use this to add extra options.
GM:AddExtraOptions(Panel panellist, Panel optionsmenu)