Felt Brougham, Lotus Prestige, Cinelli Xperience, & a Parts Bin Rain Bike
I don't know what you mean for "long distance", but I'm leaving tomorrow for a 3 day, 300 mile trip using only the Revelate Tangle frame bag and a medium size seatpost bag. I have no experience with that Gas Tank, but the Tangle is awesome.
I have one of those top tube bags. They blow. The top tube on my Focus is hydroformed, is large diameter and flat on top. The top tube bag still tilts down on either side while riding no matter how tight I have the straps.
The Gas Tank is an excellent bag. It will hold a variety miscellaneous small items that you might not want to put into your pockets. I put one on the toptube for a 255 mile ride the other day in addition to another bag. I am not sure who makes pockets large enough for food and clothing to accommodate a temperature swing from 38F to 84F. A largish saddle bag, gas tank, and pockets can almost do it. Short rides of under 150 miles are different.
I have a Gas Tank that I use from time to time, depending on the distance. It works fine (if you have the space at the front end, as hh mentions). The one warning I have is that, depending on your bike's (and your) geometry, you need to watch out for rubbing of your inside leg against the Velcro on the bag. I was using it for a while on my winter commuter (during the time of year when my commute is in the dark in both directions), and abraded my tights without knowing it. I haven't had any problems with it on my road bike, or during daylight rides (I know, it shouldn't make a difference, but apparently if I see it there, I subconsciously alter my knee position just enough to not strike the bag with every pedal stroke).
Something to think about from my experience with a Timbuk2 Goody Bag (2015). they improved on the prior year getting rid of the zipper and have it open to the side instead of back. I like it because it can hold my cell phone as well. I went with a small (5 in long about). and is the same width as my top tube at the bottom. Large enough for a few Cliff bars, cell phone, credit cards go in slots, etc. My only complaint is that while riding down a hil, the Velcro did not hold and it flipped open and a pair of glove liners blew out. Cell phone is nicely wedged into the top so it cannot fall out. Only wish thy would have put some rare earth magnets or better Velcro. It was just the one time but that is all it takes to make you question what to put in it. I do like that it has some sort of reinforcement that makes it keep its shape very well. On my Specialized Secteur, it holds on nicely.
I don't care if it's Fred, I don't care what rules it breaks, if I'm on the bike for more than 10 miles, I have my Shimano Unzen 6. I can run errands. I can stop at a stand on the side of a ride and buy oranges. I get to carry a functional pump and whatever multi-tool I want. Best of all, not a single bag on the bike.
Oh, and Shimano did their homework. The bag is comfortable and light to the point that most of the time I don't even notice I'm wearing it.