Is chant that easy? I do not know. I am a member of a parish choir and just doing the last two verses of the Holy Thursday procession hymm ( in Latin ) before the body of the Lord was placed in speical adoration alter by St. Thomas Aquintas was very hard.
Not only the notation is different - the manner of singing is different too. Let's leave aside for a moment all the little performance minutiae that the Solemnes monks have laid down, and just talk about basics.
Chant is sung as though you were speaking, with the emphasis on the words just as they would be spoken. It doesn't have a strict meter or time -- again, you chant the words just as they would be spoken, with the natural rhythm of a rather formal speaking - "declamation" as our choirmaster says. You breathe where you would breathe when speaking (the Solemnes notation kindly gives breath marks for you, they do make sense, but if you have the music in ordinary staff notation just make a tick mark where you should breathe, just as a reminder.)
As far as the tone itself, it should be a simply produced, rather "straight" tone - i.e. no vibrato or quavering, no swooping from note to note. Each note should build or flow from the previous one, from the beginning to the end of a phrase. Simplicity, solemnity, purity is what you're aiming for. If your usual music is towards the "pop" end of the scale, or the romantics, you'll have to rethink the style a little bit.
Since I came from the Episcopal Church which never abandoned chant, I was taught the basics of performance style very early on (in fact, I was instructed to "warm up" my tone a little bit because I was taught the "Anglican hoot" - which is a very straight tone with no warmth at all). But I had to learn to read the notation, which the Piskies do not use.
What you can do to practice is call up YouTube and search for it - especially the versions that run the text and music visually on the page together with the sound. Sing along. We sang the "Tantum ergo" also while processing to the Altar of Repose (actually we sang the entire "Pange Lingua", which is the longer hymn from which the "Tantum ergo" is taken. We had quite a distance to hike!)
Here's the "Pange Lingua" in its entirety, with the music flowing along so you can follow it. "Tantum ergo" begins at verse 5. The performance is very good, very simply sung with no vibrato and nice carrying tones - although their rhythm is fairly strict for chant:
The Pange Lingua? We sang 17 verses this year. I guess since I've sung it for about 25 years in a row... It's not so much that chant is hard, it's about getting the right sound.