were about to change to gmail from a webmail provided by our hosting provider, i understand changing the mx records and all. But my main worry was if there would be any emails that would fall through the gaps of the two systems during change over.

Im not familiar with the ins and outs of how the mx record works, is it like a dns record change, ie. it needs to propagate ? If thats the case would there be a period were its left my current email provider but not switched to the new gmail account ? Thus allowing emails not be delivered or worse lost ?