Well, lets see. GTK+ reference manual says:
gtk_text_buffer_insert ()
void gtk_text_buffer_insert (GtkTextBuffer *buffer,
GtkTextIter *iter,
const gchar *text,
gint len);
Inserts len bytes of text at position iter. If len is -1, text must be nul-terminated and will be inserted in its entirety. Emits the "insert-text" signal; insertion actually occurs in the default handler for the signal. iter is invalidated when insertion occurs (because the buffer contents change), but the default signal handler revalidates it to point to the end of the inserted text.
buffer : a GtkTextBuffer
iter : a position in the buffer
text : UTF-8 format text to insert
len : length of text in bytes, or -1
and
gchar
typedef char gchar;
Corresponds to the standard C char type
This means, that a char* can be used in place of gchar*.
Though I have not used gtk_text_buffer_insert() , but in other gtk+ functions that require gchar*, char* has worked flawlessly.
Hope it was helpful.