The meaning of 'verb+다는' is almost the same as 'verb+는'.
사랑한다는 걸 is similar to 사랑하는 걸 and both express the present fact. But 사랑한 걸 expresses the past fact.
'verb+다' completes a sentence.
So 'verb+다는' gives a little bit the feeling like a third person says it and it's objective statement, compared with 'verb+는'.
You can think 'verb+다는' is a verb-noun, (I am sorry I don't know the grammatical term for this.) and not subject to grammatical tenses. The English equivalent of 사랑한다는 걸 can be "It is important to learn how to love your neighbors." or "Sympathizing is not the same as loving." I assume that is why 'verb+다는' gives the feeling of the third-person narrative form.
I am sorry I should have said 'verb+다는+것' is a verb-noun. "것" makes the phrase into a noun. "'verb+다는" is an adjective, because it ends with "~ㄴ". You can see the same pattern with "사랑한" in "사랑한 걸" or "사랑하는" in "사랑하는 걸".
"~ 걸" is a shortened version of "~ 것을", which is a combination of "~ 것" + "을". In this, "을" indicates that the verb-noun is an object. In addition, "~ 것이" is a subject of the sentence, because of "이". I hope this is not too confusing.