GMLive's function definitions are based on GM's auto-completion file, which sometimes isn't entirely consistent (e.g. in this case it uses "arg*" to indicate an optional argument, whereas usually it's "[arg]").
You can update the definition by doing
gml_func_add("layer_create(depth, ?name):", g_gml_func_script[?"layer_create"]);after you call live_init.
I'll fix this in the upcoming version.