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.