Cheaters, modders and hackers are all likely the same people. The people you see cheating using mods are often not likely the ones making the mods, so to answer your question: modders (as in those who cheat by using mods) are likely not developers. Not developers for Rockstar Games, and not developers in general. Typically, the term modders refers to a player who is using a mod or cheat program (likely made by someone else).
In the same sense of the term hackers: It's not uncommon to hear the term "hackers" thrown about in any online game, GTA Online inclusive, even though the term hacker was more to describe a computer enthusiast or one who hacked together programs for fun. Wikipedia says:
Today, mainstream usage of "hacker" mostly refers to computer criminals, due to the mass media usage of the word since the 1980s. This includes what hacker slang calls "script kiddies," people breaking into computers using programs written by others, with very little knowledge about the way they work. This usage has become so predominant that the general public is unaware that different meanings exist.
Those who use cheat programs in games are coined hackers or modders rather than just cheaters. While I can't find any evidence for this, I would assume it's to point out the exact way they are cheating.
For example, let's say you and I wished to have a race in GTA Online and we decided on some rules; however, I broke those rules within the context of the game (cut corners, took a shortcut, brought along a modified car etc) in order to benefit myself and win, you might call me a cheater.
If instead my car started ignoring gravity, or I was invincible or faster than a car can go by the rules of the game, you would probably consider me a "hacker" or "modder" because I'd be (most definitely) using some kind of program to alter the game.
My advice to you is to simply ignore these people. Don't fuel their twisted sense of fun by typing in chat; instead, report them! I know it might feel like nothing ever happens, but you might as well do everything you can.
Rockstar have announced that they are targeting cheaters:
As part of our ongoing commitment to combat cheating and griefing, we have recently implemented new systems to allow us to more efficiently detect and target any players found running mods and cheat programs within GTA Online.
A link from the same page details how to report cheaters:
- Pause the game and navigate to the “ONLINE” tab.
- Select “Players”, and choose the name of the player cheating.
- Select “Report”, and confirm your report with the “Exploit” category.
Rockstar suggests that this is the best way to report cheaters, rather than forums etc:
The best and most effective way to report cheaters in GTA Online is through the in-game menu. When reports are sent to us via the in-game tools, we are able to study much more data to help identify and act on the user and the activity in question
It is also possible kick players from the same menu you use to report them. I can't find numbers confirmation, but a few votes from players on the server should at least kick the offending player from the server.