GMDX has a huge number of gameplay and balance changes. Here are some of them: https://deusex.fandom.com/wiki/Give_Me_Deus_Ex_(Mod)
https://videogamegeek.com/thread/1831032/review-of-gmdx-mod-which-aims-at-modernizing-deus
I can try to grab some excerpts without spoilers...
GMDX offers new player mechanics, refined AI behavior, reworked weapon mechanics, rebalanced RPG elements, enhanced artistic direction, expanded level design, overhauled audio design, new interactions and simulated effects, improved user interface, and hundreds of bug fixes.
Messing with the design of Deus Ex may quickly hurt the experience. And that is what GMDX does.
In early areas, the changes that GMDX makes are minor, which is the reason for my early positive feeling on GMDX. In later areas, however, there are radical changes. The aim seems to have been to increase the difficulty. And it succeeds at that: the difficulty of GMDX is certainly quite a lot higher than the difficulty of Deus Ex. But higher difficulty does not make for a better game. That is particularly true because of the reasons why the game is more difficult with GMDX.