In some versions of this quote, the word "attack" is used in place of "fight."
To attempt to verify Gandhi as a source, I would recommend checking out The Mahatma Gandhi Research and Media Service site link to site.
The site has a browseable full text version of Gandhi's 100 volume Collected Works at Collected Works . A simple search feature will allow you to search the full text of the collection, as a free-text search.
There is also a browseable list of contents, which include letters, cables, telegrams, and notes. The tone of the quote suggests it could have appeared in something like a letter.
You may try keywords within the quote. In the Collected Works, there are numerous quotes referencing laughter with similar ideas (as in, the concept of nonviolent resistance meeting with laughter from those in power). It is possible that the quote is a paraphrase in which the paraphrasers took some serious liberties with the original.
If all your searching proves frustrating, you may also wish to consult a Gandhi scholar. There is information on Gandhi studies journals on the main page of the web site. Just look for the link to "Journals." As this alleged quote has been widely used, perhaps Gandhi scholars would have some insight into its origins or possible misattributions.
Books you may consider reading, should you wish to pursue this in depth, include ones where Gandhi focuses on discussions of non-violent strategy (as the quote suggests this theme). Try:
Non-Violence in Peace and War
Non-Violent Resistance (Satyagraha)
Satyagraha in South Africa
Best of luck!
Vani