It was me who asked the question on the Gandhitopia forum that you found. I have also been unable to find a source for the quote the homeopaths attribute to Gandhi.
However, as you could see, I found those quotes from his letters recorded in what appears to be a reliable source (http://www.gandhiserve.org/), but they tend to indicate he was not a fan (the later quotes are first):
14.10.1941 I have little faith in homeopathy and yet a homeopath a crank like myself is here and treating some patients.
11.07.1940 (1) Disapproves his asking money from public Fund. (2) One needs less money if he keeps himself physically fit. Says he (G) has arranged for his Homeopathy treatment at Delhi. Hopes his total recivery.
04.04.1933 Though I have no faith in Homeopathy you may introduce the treatment after consulting the people there:
28.04.1933 Gandhiji asserts that personally he would prefer Homeopathy to Allopathy any day. As such himself does not mind his trying it after due discussion with Narandas.
16.04.1933 I have not much faith homeopathy.
For a few of these, it's difficult to know what his tone was and whether he was being sarcastic or flippant. Also, the longer quote looks like a summary of a letter, so not exactly what Gandhi wrote.
Unfortunately, no one else there replied and I have not been able to find out any more. So, as far as I'm aware, the only sources we have seem to indicate overall that Gandhi was not a homeopathy believer. I suspect the homeopaths' quote is fantasy.