I think the forward slash is used as an escape sequence to denote that the slash in the block name is part of the name, to avoid confusing it with another subsystem level.
For example, if I have a block named 'b/c' in a model called 'a', gcb will return 'a/b//c', because if it returned 'a/b/c', this could be interpreted as a block called 'c' in a subsystem called 'b' which is inside a model called 'a'.
Why would you like to avoid the escape sequence? I think you will have trouble using something like 'a/b/c' to denote the above described block, because Simulink won't be able to find a subsystem called 'b'. In any case, if you still want to remove the escape sequences, you need to do some string manipulation yourself. One simplistic way of doing this is: strrep(gcb, '//', '/'), but this doesn't handle multiple slashes, like in 'a/b////c'.
