GMs aren't useful for much else, so the risk of unbalancing the rest of the strategies is limited. If we filled this new spammable building with GAs everyone would have wanted them for perma-golden ages. So let's not do that.
Also, if GMs spawn early then the tourism they generate is fixed early. So you won't be saving up ten GMs for a mega culture bomb in the info age; if you choose to start generating them in the industrial you'd better win by then or in the early modern era or you're screwed by exponential costs.
By adding a new industrial era building, we're forcing people to choose between building this first and building factories. And factories mean an ideology. Choices, choices...
There are lots of other levers we can pull to fine tune the game balance.
The Hotel, Airport, International Visitor Centre and Internet all give mega tourism bonuses. If adding the Concert Hall makes cultural victory too easy, we can turn these down, as hard as needed.
The big risk that I see is people holding back building the Concert Hall until they've got Hotels, Airports, etc, so each GM gives mega tourism points (as now) and they can suddenly flood the world with EMI's entire repertoire. But this becomes much less attractive if we turn down the bonuses from the tourism boosters: people would be sacrificing far more turns of GM generation than at present (because it's essential N turns multiplied by how many Concert Halls) in return for a reduced late-game boost.
Another lever we could pull is to say each GM requires Maple, not just Concert Halls. That will disincentivise keeping an army of them hanging around unspent. You'd need to give the first
n GMs maple-free, otherwise maple becomes
essential for tourism victories, which is undesirable.
On the other hand this is encouraging tall cultural players to go out, settle more cities, possibly go to war to settle, another building to build (and if you want it early, it's just when you need to get those factories out to grab an ideology), reveal your escalating tourism potential, and suffer all the AI jealousy that comes from all of that. Perhaps the Concert Hall will need boosting, not nerfing, so that people build it at all: in particular it may need to generate a few culture points to off-set the extra cities that may be needed to grab extra maple. If necessary, boosting the Concert Hall with some fat

generation will further discourage people from artificially holding back from building it, and will give non-tourism players reason to build it (and so reason to compete for maple).
I'm assuming the map generator can also control the scarcity to some extent the distribution of maple.