If it's an ASCII drawing, you need a text editor, not Gimp. Any decent text editor that supports 1) fixed-width fonts and 2) block-selection (Notepad++, UltraEdit...) will do.
On the other hand, this is 2018, last time I have seen ASCII-based ER diagrams like this was when we were all working on the Y2K problem. There are now plenty of modeling tools that will produce such diagrams, but much prettier. Many will even do so automatically if you give them access to the DB.
On the other hand, this is 2018, last time I have seen ASCII-based ER diagrams like this was when we were all working on the Y2K problem. There are now plenty of modeling tools that will produce such diagrams, but much prettier. Many will even do so automatically if you give them access to the DB.