yes, could you do a field extraction for those values?that way they could be under the same FIELD. then try this: 
 sourcetype=gc.log FIELD="PSYoungGen" OR FIELD="Full GC" | stats count(FIELD=="PSYoungGen") AS "GCs", count(FIELD="FullGC") AS "Full_GCs", count AS "Total" ...... and then calculate your percentange  
 I was assuming that in some events you got this value "PSYoungGen" and in some other you got "Full GC" , if it does not work like that maybe you can still see where I was going with my explanation. Please do not hesitate to ask if you don't understand 
						
					
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