gtf 1280 1024 60 will do that for you.
However, seeing as you have gtf to begin with, you therefore have X 4.3.0. Right?
If so (well heck, even if not), you can just tell it to use 1280x1024, and then make sure your HorizSync and VertRefresh lines have the correct ranges for your monitor (check the monitor documentation for the correct ranges to use), and it'll use the highest refresh rate it knows about.
Hang on... what happens when you try 1280x1024? It may very well not be the case that you need to write a modeline, but it depends on the reason that mode is getting thrown out -- what's in your X log when you try to use that resolution?