Gunships need:
- The speed to run the opponent down. Even at the very highest tech, beam weapons are limited to 1.5 m-km range, so you need to chase your opponent down.
- Armor (and/or shields) to withstand multiple hits. Against a missile-armed enemy, you might get hit a dozen times while closing to gun range.
- Cloaking and/or ECM to evade incoming enemy fire, and ECCM to penetrate or bypass enemy defenses.
Gunships are indeed awesome, but they require high tech in several fields... engines, thrust augmentation, armor, shields, ECM, ECCM, Cloaking, Lasers, Power, Capacitors...
Of course, you don't need to wait until you have all of that before building Gunships. Just be aware that some opponents will be impossible to chase down, or will hammer you if you leave your PD screen, etc.