Gforth is case-insensitive for ASCII characters and case-insensitive for non-ASCII characters. I.e., you can invoke Standard words using upper, lower or mixed case.
For now, standard Forth only requires implementations to recognise Standard words when they are typed entirely in upper case. You can use whatever case you like for words that you define, but in a Standard program you have to use the words in the same case that you defined them.
Gforth supports case sensitivity through cs-wordlists (case-sensitive
wordlists, see Word Lists).