Hello,
It is a period when I open Gcedit this message appears to me.
I do not understand what I have to do to eliminate it.
Note:
it only makes me on that computer, I tried to uninstall and install Mach4, but to no avail.

There seems to be an issue when using 'Match Case' option when using Find or Replace functions in gcEdit.

Open the following file, where <account name> is your Windows user account name
C:\Users\<account name>\AppData\Roaming\gcEdit.ini

Scroll down to MatchCase=4 
Set MatchCase=0

This issue will reappear if you select 'Match Case' again. The match case function still works, its just an annoying message box

Yes!!
solved as you said!
 thanks!!