Seeking alternative GUI File editor with large file support

Try using hexedit I haven't tried it on HP-UX but it should work. It allows you to move to a location in a file and truncate. I'm pretty sure that it does not read the whole file in but just seeks to the appropriate location for display.

Usage is fairly simple once you have launched it the arrow keys allow you to move around. F1 gives help. Ctrl-G moves to a location in the file (hint: to move to end use the size of the file from the bottom row of the display). Position the cursor on the first byte that you want to truncate and then press Escape T once you confirm the truncate will have been done. Ctrl-x exits.

This may not be exactly what you're looking for, but hexedit will open large files like that. It only has what's displayed in the buffer (and maybe a little bit more) in memory. It's made for editing raw disk files (e.g., /dev/sda) and will display in hex side by side with ASCII or EBCDIC.