Englishal
I have sen the same problem with filler caps on, where the rubber seals had perished. This is common in flying school planes, particularly PA38s. I once drained out about a pint of water (!!) after a night of heavy rain.
Re the fuel totaliser, one needs a flowmeter, e.g. a Shadin, coupled to the GPS. The GPS gets an RS232 data stream from the flowmeter, giving the flow rate and the remaining fuel, and it projects the reserve at the flight planned destination. It's a nice feature, though not essential to flight safety because I always fly with at least 2hrs reserve. I suppose it would be handy after a series of emergency weather diversions...
I don't think a GPS can do this alone (without a flowmeter) - certainly it would not be accurate because it won't know the flow rate.