If you look at real world scenarios all terrorist attacks really do is rally the people for the cause. In the real world there has been little problem bringing in weapons, or improvising. The problem is getting the bomb in place where it could do some damage, or acquiring the intel, or the many other problems that a simple security system causes that magically wont go away with a teleporter, maybe the antigrav can create a detector that picks up the unobtanium that makes the teleoporters work, maybe some anti terrorist agents can sniff out the plots and catch the operatives.
The antigravs can use their own spies to do the same in the teleporter nation. They can use antigravity canisters, in cunjunction with High altitude stealth uavs to smuggle in and out packages that can be potentially considerably bigger than what the teleporters can do even through radar cover.
also a quick recap of the OP and we get this:
first of all none of them have atomic or biological weapons
which means no anthrax or other forms of biological warfare.
Here is an idea in the 1940s the Germans built a large cannon called the pairs gun. this weapon was able to launch projectiles across the Maginot line and hit paris, while not significantly damaging the psychological impact of such a weapon was note worthy. in the 1980s Iraq also designed similar weapons but never had them constructed. Along with the Iraqi weapons there was also projects harp and babylon designed for high atmosphere projectile launches as a cheap way to test re entry vehicles.
The concept of using the superguns is that while missiles may be able to have similar or longer range the projectiles from the cannons are significantly harder to detect. If the antigrav nation decided to build one of these guns and mount it on one of their airships you could have a high altitude artillery cannon that could bomb the enemy from the safety your own nation, and seeing as most all weapons that could reach those heights are easily detectable destroying such a weapon would be difficult, even attempting to sabotage the ship would be extremely difficult, like trying to make an aircraft carrier sink. Sure the ship would be slow but who cares your in friendly territory the entire time, plus none of those pesky enemy radar arrays to detect them.