"traveling is tiring."
No, you can't, so this English translation is somewhat misleading, though I can understand since no one says "Travel is tiring." This is a good example of how streamlined Korean can become, since this is a natural way to say something that could be potentially long in English "Traveling makes me feel tired."
To make a verb a gerund, you can do it in two ways. First add 은/는 like you said and then add 것. 맥주를 마시는 것 = drinking beer. 친절하는 것 = being friendly. 먹는 것 = eating.
The other way is to add ~기 to the verb stem. 마시기, 먹기, etc.
By Duo's lesson notes, which agree with your post, "Traveling is tiring" should be "여행은 것이 피군해요" or "여행하기 피군해요" which are both currently, as of 2018-01-16, incorrect. The Korean sentence that is marked as correct, in my opinion, really translates to "Trips are tiring" which is a very similar, but certainly different thing.
A lot of English verbs actually start out as a gerund in Korean. So the basic word for travel in Korean is 여행, which means 'traveling', and is already a noun. To make it a verb you need to add something else, 여행하다 (to travel), or 여행을 가다 (to go on a trip), etc. 은/는 is just added to signify that traveling is the sentence topic