As far as the grinding- this trans has no synchros- try putting the trans in neutral with the clutch depressed, engine idling, for about 20 seconds. Then try putting in forward or reverse & see if it grinds. If it doesn't there's no problem, if it does, then the clutch disc is binding in the pressure plate assy & further diagnostic work will be needed.

As far the the lpg hissing, these older units usually use an electric shut off solenoid assy for the fuel which gets its power source directly from the ignition coil. If the lpg system is anything but Impco the fuel will flow right through the regulator & carb & come out the air cleaner assy if the ignition is on but the engine isn't running. Installing a vacuum actuating valve (mounted on the intake manifold-wired in between the ignition coil & lpg solenoid) will solve the hissing noise when the ign. key is on & the engine isn't running.

As far as the milky oil, there are multiple reasons that may happen (crack in head/block, faulty head gasket, etc) but the most likely cause is that the engine isn't run long enough to cook out the condensation in the crankcase. I would change the engine oil (use SAE 30), make sure a 180* thermostat is installed & run engine at least an hour straight on a weekly basis for a month. After a month check to see if the oil is turning milky. If it is, check the level on the dipstick very carefully. If the level seems to be creeping up the dipstick, there is indeed a crack/ faulty head gasket present & further diagnostic work will be needed