I'll answer your original question, but before that I'd like to point out that even though there are sometimes many ways to get something accomplished, doing it 'by the book' is sometimes better since you may not always be the person administrating a system. I've used /etc/rc.d/8/userdef and even /etc/rc2.d/S99route 'methods' to add routing information at boot time, and other admins miss it because they are looking for the 'documented' methods. But then leave it to SCO to keep changing the 'documented' method between releases...
Under SCO OpenServer 5.0.6 Enterprise, you can edit the /etc/default/tcp file directly. Since you are working on a unix system, I _strongly_ suggest you at least learn the basics of using the 'vi' (or 'vim', if you will) editor. This editor is found on just about every unix platform in a base install and is going to be your way around editing files. The commands are a bit cryptic to learn, but simple enough.
You should be logged in as 'root' to do the following steps to edit your tcp file, typing the commands in RED . Do NOT press the Enter key unless stated.:
1. Login as root.
2. When you get a # prompt, type the following, vi /etc/default/tcp and press Enter . This starts the 'vi' editor with editing the file in question.
3. You should now be looking at a file on your screen, press the letter j (lowercase) until your cursor is on the line starting with GATEWAY=. You can also use arrow keys, but h=left, j=down, k=right, l=up if a terminal emulation doesn't support arrows. So j moves your cursor down a line at a time.
4. Press the letter l (again, lowercase) until your cursor is on the 'equals' (=) symbol, then press the letter a. Type the IP address of your gateway, then press the Esc key. You are using the letter l to move the cursor to the right, then pressing a 'appends' after the cursor in edit mode. Pressing Esc exits edit mode.
5. Type the : character, then type wq and press the Enter key. The : starts a different type of vi command, then the w means write the file, the q means quit the editor.
6. You should now be at a '#' prompt again, your /etc/default/tcp file edited to set a default route at boot time.