When i look up information about Massilia I find almost no information on it.I find it surprising that Massilia was able to have success in Gaul seeing how i would normally think a wealthy foreign city would be a prime target for the local barbarians. How were they able to maintain independence and even expand in such an isolated area of the Greek world, how large was the population of the city, what did they trade and with who, what did their battles and armies look like. I'm mostly interested in the time period before Rome started having a lot of influence over the city.
I highly and strongly recommend that you look at Michael Dietler's excellent book, Archaeologies of Colonialism. Even though Dietler is making larger, more theoretical points about cultural interaction, he uses Massilia extensively as a case study for how culture changes. He starts with the pre-Greek sites there, discusses the Gallic cultures as they survived in the archaeological record, and proceeds with Greek colonization, the trade economies of Massilia as being a major trade hub in the western Mediterranean, and continues throughout the Roman period. The book is an excellent example of a diachronic, multi-faceted historical investigation of an ancient site, using archaeological, textual, and historiographic evidence to do so. The book has received many awards from various academic institutions, so I can not recommend this book to you enough.