@axehaxe,
they'll already get the maximum RoP reputation hit if they attack you while in your territory and without war declaration. Meanwhile, if you give it to them, more of your cities will be in their reach (especially if you have rails), and they may decide to change the attack target and go take a lightly defended city of yours first. As only the first attacker will be able to use your road network this may not be such a big problem unless you do like me, i.e. defend mainly the border cities and keep most of your empire empty.
And to the second question, if you capture the GL and you don't have education, it'll get you all the techs eligible (known by 2 civs). But if you have education, all you can get is some more commerce later (tourism) that pays for higher flip risk... There are quite some SGs that have used this, some of them go up 2 ages when they capture it.