1. The 'Prop-Shield'
When facing Combine Soldiers, especially in open areas, use the Gravity Gun to pick up a large, sturdy object (like a metal sheet or a wooden door) and hold it in front of you as a mobile shield. You can then peek around it to fire or use it to advance safely.
2. Environmental Explosions
Always be on the lookout for explosive barrels, gas canisters, or even propane tanks. Use the Gravity Gun to pick them up and launch them at groups of enemies for massive area-of-effect damage, conserving your precious ammunition.
3. Disarming Turrets
Combine turrets can be a nuisance. Instead of shooting them, use the Gravity Gun to pick them up and either turn them against their allies, throw them into a pit, or simply disable them by knocking them over. This is especially useful in tight corridors.
4. Headcrab Launching
In Ravenholm, you can pick up live Headcrabs with the Gravity Gun and launch them at other enemies, including Zombies. This can be a morbidly effective way to deal damage and create diversions without using ammo.
5. Supercharged Gravity Gun Mastery
Once supercharged in the Citadel, the Gravity Gun becomes your ultimate weapon. Practice grabbing Combine Soldiers and launching them into environmental hazards or off ledges. You can also grab their own energy balls and fire them back, or even rip off armor plating from Striders. It's incredibly powerful and fun to experiment with.