I don't know the precise formula for 1920's gramophone, but this is the general process for audio antiquing and should get you started. Depending on the exact settings, you can get vinyl, telephone, etc.
- Convert to mono.
- Add white noise.
- Bandpass. I would start with a HP at 80 HZ and a steep LP at 8 kHz, but that's a guess. You'll have to use your ears until it sounds like the gramophone. You should try Chebychev and Butterworth filters if available.
You may also want to add "clicks" and "Pops". You can model those in a variety of ways. One way to start is with a Dirac function and filter it with bandpass functions. Depending on how strict your bandpass in step three is, you might be able to just add the clicks and pops as Diracs at the same time you add white noise, but then all your clicks and pops will sound about the same, which isn't very realistic.
Some other things you may want to try for more realism:
- applying compression and saturation after step 1.
- applying wow and flutter emulation either before step 2 or after (or, for most accuracy, two stages of white noise before and after the wow and flutter emulation).
You may want to ask on http://avp.stackexchange.com/ to see exactly what filter settings they would recommend for 1920's gramophone in particular.
For tips on implementing basic audio filters, I have a blog post: http://blog.bjornroche.com/2012/08/basic-audio-eqs.html