There are a set of constants defined in Borderlands, with names like GD_Itempools.Scheduling.GameStage_02. These are used by the game to restrict what loot is available at what levels, which is how you only have pistols at the beginning of the game, etc. The number at the end of the object (ie: the 02 in GameStage_02) matches the level that the constant matches, so gear which has a MinGameStageRequirement of GD_Itempools.Scheduling.GameStage_15 will only be available in levels whose Game Stage is at least 15.

The actual constants can be tweaked by modding, however. UCP, for instance, sets the 02, 03, and 07 levels to 1, which allows all weapon types (minus launchers), grenades, and Eridium right from the start of the game.

It's worth noting that these mostly just unlock weapon/item types and rarities, but are not used to determine when things like weapon accessories and elements are unlocked - those are basically hardcoded in each item/weapon's parts list. This applies to class mods, grenade mod delivery systems, grenade mod payloads, when rocket+grenade ammo can show up in vending machine's, etc. Apocalyptech's BL2 Early Bloomer and TPS Early Bloomer mods unlock basically everything from the beginning of the game, so see those for reference, if you're curious where everything else is.

GameStage	Unlocks
02	White and Green Pistols Dice Chest "low" pistol rewards
03	White and Green Standard Shields (shields might not actually unlock until 04) some skin rewards
04	Main Standard shields pool
05	White and Green Shotguns + ARs Spell Grenades the other Dice Chest rewards Gwen's Head
06	Grenade ammo
07	All weapon (minus launchers), standard shield, and grenade rarities Eridium Seraph Crystals
08	Turtle and Spike shields (all rarities)
09	(nothing)
10	Nova Shields (all rarities) (Nova shields might not actually unlock until 12) Rocket Launchers (all rarities)
11	(nothing)
12	Corrosive, Explosive, Fire, and Shock Nova Shields (all rarities) Maylay Shields (all rarities)
13	Booster Shields (all rarities)
14	Absorb Shields (all rarities) main Relic pool (Relics might not actually unlock until 15)
15	Relics (all rarities) Amp Shields (all rarities)
16	Adaptive Shields (all rarities)
17	(nothing)
18	(nothing)
19	(nothing)
20	(nothing)
21	(nothing)
22	(nothing)
23	(nothing)
24	Used to determine whether story missions give Backpack SDU rewards

GameStage	Unlocks
02	White and Green Pistols
03	White and Green ARs White and Green Standard Shields (shields might not unlock until 04)
04	White OzKits main Shield Pool
05	White and Green Shotguns, Snipers, and Grenade Mods main Lasers pool Gwen's Head Moonstone (lasers might not drop until 06)
06	Green OzKits main Grinder pool (grinder may not be active until 07) Blue weapons (minus launchers, including SMGs (note that white+green SMGs cannot drop at this point)) Shift weapons (Boomacorn, Jack-O-Cannon, The Machine, Ol'Painful, Contraband Sky Rocket)
07	Grinder Blue and higher grenade mods, standard shields, and weapons (minus launchers and SMGs) Glitch weapons Many other Claptastic Voyage pools
08	SMGs (all rarities) Blue OzKits Spike and Turtle shields (all rarities)
09	(nothing)
10	Launchers (all rarities) Nova Shields (all rarities) Some DLC Grinder Recipes
11	(nothing)
12	Purple OzKits Maylay Shields (all rarities) Ice, Corrosive, Shock, Fire, and Explosive Nova Shields (all rarities)
13	Booster Shields (all rarities)
14	Absorb Shields (all rarities)
15	Amp Shields (all rarities)
16	Adaptive Shields (all rarities) New-style Hyperion Weapons (all rarities)
17	(nothing)
18	Legendary OzKits
19	(nothing)
20	(nothing)
21	(nothing)
22	(nothing)
23	(nothing)
24	Used to determine whether story missions give Backpack SDU rewards