You are exaggerating a little bit, don't you think?
I can list those fields were they equalled and even surpassed the romans :
-philosophy & theology : the druids weren't only religious leaders. In fact, like many civilizations of that time, the priest and scientist class were the same, look at the egyptians, the persians, the mayans... They were great philosophers, even the greeks and the roman praised the deepness of their thoughs
-Litterature : the fact that they didn't write doesn't mean they had not complex litterature. In fact their poems, essays and stories were transmitted orally. Even today most people have heard of the bards, which is a testimony to their prowess in this fields. They capable of Literary techniques that would have put sometimes some of todays writer to shame. Their litterature wasn't just instinctive.
-Craftmanship : it's one of those fields were they were often (not always) surpassing the romans. Everything ivolving smithing or building objects (made of woods, or other materials). They were by far the finest metal workers in antiquity, smithing being very complex in nature, this is a testimony of their overall level of advancement. They invented the mailcoat (that revolutionized armament ), the plow (that indisputably revolutionized agriculture) and had left a lot of very fine artworks, most of them having only been recently uncovered. They also invented the barell, one of the most ingenious way to transport liquids and other goods.
-MAthematics : yeah, now i know most people are surprise and often don't believe it when they hear this, because it's really the antithesis of today's cliche image of the gauls. But the druids did travel a lot, goign to place like alexiandria, the capital of sience at the time (great library) to discuss and exchange theory with the others sicientist (greek,romans,egyptians sometimes even indians). What we know for sure is that they had the same knowledge of those people. What we don't know yet is if they discovered theorems themselves. Since they did not leave any written notes, it's hard to tell. So I'm going to assume theey didn't had a big impact on mathematical theories and were just educated, just like the romans : the romans didn't really liked abstract thinking themselves, so both gauls and romans are probably equals in those fields.
- Art : it's difficult tho judge art, and I think you can never say one painting is superior to another. You can't judge creativity But, you can judge the level of complexity, the level technique used, and the overall finesse . In those two, the gauls were equals to the romans.They made extremely fine piece piece of art, mainly using metals and wood, while the romans prefered marble and stone. I've many books with lots of pictures about the art of the two civs, and myself i can tell they they are both equals when it comes to the finesse.
In the fields of architecture, the romans surpassed the gauls, like they surpassed everybody anyway.
But that doesn't means the gauls had a primitive architecture. In fact they had a very unique style.
Again contrary to the popular belief, they did build more than just huts and longhouse. Their cities were fully paved, like their roads, their road systhem was one of the most develloped and well kept of those time, with every roads being made of stone or woods. Only the poorer lived in huts, and most of them had big houses, improved from generations to generations, with fine artwork all around the house, low relief, sculptures, and paintings. The cities had big administrative buildings, again with lots of artwork and decorations, large public facilities (even baths, although not as extended as the romans ones), large central places...
If you want to picture one of their largest oppdiums (cities), just imagine any other big cities of that times (carthage, sparta, even rome) just a bit smaller (there was a bigger rural population) and replace the stone and marble by a wood/stone combo and metal working : you have your gaul oppidium, the same grandeur, the same imposing feeling (or almost, let's not exagerate )