And you have to research aperture/strength/damage for this, not range related techs like wavelength.
Note that it seems that only lasers can get you to the highest end-tech rating, because other technologies don't have enough levels. Although that may have changed in 2.0+, I am not sure.
Specifically, only Laser, Railgun, Plasma, and Particle Beam techs will improve ground unit attack. Missile, Gauss, Meson, and HPM will have no effect. Honestly this is stupid, is effectively a nerf to the latter weapon types - as if missiles or Mesons needed any more problems - and I prefer the VB6 method of having an actual tech for GU attack (which, as a DB modder, would also let me actually rebalance ship armor without affecting ground units adversely).
I've got to say I approve of the idea that ground and space weapon+defense tech be connected. What weird situation would it be if one empire somehow had invincible soldiers and tanks, but was still broadsiding enemy ships with handheld culverins?
The implementation might be tricky, and I agree that all weapon techs should be able to contribute. The problem with the tech lines not being equally long definitely has to be solved, and perhaps shield technology could also play a role in ground defense.
Maybe the advance could just happen on a "Y-tech points in finished weapons/defense technologies for next upgrade" basis, though I am not sure how to keep that smoothly and intuitive.
I am thinking about it this way, because I realized that you couldn't patch out the tier-length issue by simply letting the shorter trees give more. (eg "Particle Beam-2" might get you to strength 6, but "10cm Laser" only to 5. All adjusted so in the end both reach max GU strength) If you did that, then suddenly some tech lines would be at an advantage due to reaching max GU-attack at smaller cost. ...Which is why I came to think that maybe you could just lay out the tech points needed for max GU-attack and then let all techs contribute evenly, so you can kind of 'augment' the shorter techlines after they reach the max through some help.
All this would make the leveling much more opaque however. I am not confident in the good feel of that solution, but I
do still feel GU weapon strength should be tied to actual weapon tech like it is now.