It certainly makes for more disclosure, but how big a deal it'll be depends on how much people pay attention and care. Given the way e.g. the Carrier boondoggle played out, I wouldn't hope for much.
GASB is a private standards-issuing organization, not a govt agency, so there's nothing Trump/Congress can do in terms of direct repeal. But not all states require local govts to follow these standards, and those that do could repeal that requirement if they wanted to I suppose.
For a long time there was very little public accounting of federal tax expenditures like the mortgage interest exclusion. Now there are, but it doesn't seem to have done much. But maybe it'll be different given GASB applies to state and local levels.
Ex recto, it looks like it'll strengthen the party in power, whichever one it is. States with Democratic governments can follow the guideline, acknowledge the huge tax breaks they've been giving to businesses for no good reason, and maybe whittle away at them a tiny bit if the business in question is already unpopular, pleasing their constituents both on principle and because their government is now better funded. States with Republican government can ignore the guideline (or, per 1, repeal the law that requires them to follow it), continue to hide the tax breaks, and their constituents can continue to vote against their interest in blissful ignorance.
Well, that makes it sound useful in battleground districts.
I don't know why "Ex Lax" wasn't named "Ex Recto," but I'm sure the reason reflects poorly on the functioning of American business.