Correct. The ship cap is 49 on normal (98 on high!), I checked. There is no optimally balanced gravity range because of the following reasons:
A. The gravity effect does not scale with ship cap. Even though a gravity drain on high is much weaker than on low, the gravity effect is still the same (8000/17 speed max) for both, so on high both the player and AI can completely nullify an entire planet with proper placement. With 98 mk1 gravity drains with 8000 range, and proper placement, a player could essentially slow an entire enemy wave across the entire approach (possibly up to 20 planets, if we're assuming a maximum distance from one wormhole to the next at 40,000, which seems reasonable) to the player homeworld to 17. Add in the mk2 and mk3 (maybe even mk4) gravity drains at 88/78/68 ship caps IIRC, and a player can essentially shut down all free AI movement in the entire galaxy. That the AI is not limited by ship caps make this an almost impossible proposition for players to deal with, especially with the preponderance of mk3 and mk4 gravity drains they will have to face.
B. Even if the range were lessened so that players cannot do this, the AI is still not constrained by ship cap, so the player will most likely still need to kill every gravity drain they come across, because once a human fleet is within a gravity drain's AOE they will never be able to leave unless they kill the gravity drain, or manage to micro out the specific targets the gravity drain is attacking. This can be a daunting task when the AI has 40 of them on a planet, which is possible even on low ship caps, and even worse on high level planets where speed is going to be reduced to far less than 17.
Gravity Drains should probably be moved into the range of .05 ship cap, with no scaling for high/med/low ship caps because their main utility (the gravity effect) already does not scale. Their hitpoints and damage should probably not be increased 10-fold as the ship cap decrease would otherwise suggest, because that would make the destruction of even one gravity drain a daunting task. Rather, they should be relatively easy for players to kill if used by the AI once the gravity drain is in range (they will still be a problem because the huge slowing effect will let the AI fleet absolutely trounce any human fleet wandering around in-system). This will not largely affect player usage of the gravity drains because of how the large range, and a screening fleet, will still allow even a small number of gravity drains to affect most of a system. Adding in the speed increase should ensure that players are not impossibly stuck within gravity fields for too long, and will make gravity drains a slightly different issue (and easier to deal with, individual) compared to Gravity Guardians.
Edit: Optimal speed would probably be somewhere in the 56-68 range. They should be slower than the main fleet ships so that on the AI side they cannot catch up to a fleeing group of fighters/bombers (which would be disastrous), and so that on the player side there is a consideration in throwing them in with the main fleet ships.
A player combining gravity drains with fleet ships should be a particular balance point to be concerned with, since the combination will largely allow a player to move his fleet around within AI territory without having to deal with too much resistance from AI defensive responses that have a weapons range of less than 8000 (except for whatever is at the player's ultimate destination).