Okay, so i got this CHAT GUI, me and my friend made it, mostly by googling stuff, and the finished result looked good, until we started typing in it, it censors more than the roblox original chat and i cant seem to find whats going on.
By any chance did you just use a free model one? If its completely custom then it might use a custom filter meaning that there will be a list of words somewhere that it will censor.
Quite the vague question. I happen to of made my own custom chat from scratch. The method I used was.
FilteringStringForBroadcast
However, I would like to say that this is a built in function and I highly doubt ANY SCRIPTER would or at least should ever make their own API for filtering, for one, it will get taken down by ROBLOX if it is not absolutely perfect due to ToS. Per that fact, I find it hard to believe anything is wrong with your ROBLOX chat, check your code again, and loop for what censors your string, and if it is FilteringStringForBroadcast, then it should work fine.
A little tip is when going into the script, press CTRL + F and search for the built in functions you are looking for. This takes into account every string in the script, and returns if anything matches that string.
If you want to search your entire game however, go to your game window(Not in any script) and do CTRL + SHIFT + F
I hope this helped you, but I'm afraid that if your filtering is messed up truly, then it's best not to have that system in your game at all and redevelop it.