SY M28 is a helluva place to start i'd say value if in good condition is at least $400 maybe a little less if it has been upgraded to a M28/30 (look at the rear sight if it looks like
this it's an M28 if it looks like the one on my M39 it's a 28/30)
if you want a bayonet for your rifle... good luck... they don't pop up often and are not cheap
when you say gummy i have to ask... have you ever fired or cleaned this rifle? wipe a white rag across the metal does it come back yellowish orange? it's covered in cosmoline... to remove cosmoline you need heat
for the barrel get a funnel a bucket and a kettle take the rifle apart and with the stock where it won't get wet hold one end of the barrel over the bucket with the funnel in the other (thick gloves are a good idea... barrel will get hot) have someone pour boiling water into the funnel... the heat of the water will soften and melt the cosmoline and allow it to flow freely... repeat at least once while the barrel is still warm wipe excess cosmoline from the barrel exterior and clean bore as normal
for the small parts you can choose to bake them at low heat in an oven or boil them in an old pot (i keep one i bought at a rummage sale just for such purposes) dry all parts immediately and you won't have to worry about rust
for the stock wrap it in newspaper and sit it someplace that gets hot (dashboard of a car on a hot summer day) let it sit all day bring it in wipe it off if you want you can then hit it with a hair dryer
it's next to impossible to get all the cosmoline initially (especially from the stock) expect the heat from shooting to cause more cosmoline to bleed out slowly
and last but not least always clean immediately after firing if using surplus ammo and always use water or a water based solvent for the immediate cleaning (i like to do a quick pass with a water based solvent (good old usgi bore cleaner.. not the vietnam and later stuff in the plastic bottle but the ww2/korea stuff in the can) at the range efore packing up to go home then do a proper cleaning later that evening)