Currently in TW, if you barely lost, then you may as well have set defense, not tried at all, and took the day off, because the rewards for a close loss are the same as not attacking at all. But how can you tie effort to rewards, and thus motivate guildmates to attack? Especially in a way that prevents gaming the system?

Make TW rewards tiered based on the difference between your guild's final score and the opposing guild's final score.
E.g. winning by +500 points or more moves you up a GP reward bracket, and your opponent down one bracket. Winning by +2000 points could move you up two GP brackets in rewards for that TW, and your opponent down by the same amount.

(The rewards have to be based on the difference between the two scores and not the actual scores themselves; you wouldn't want to motivate people to set weak defenses so they can get a full clear on the enemy.)

(Cross-posted on Reddit)
SWGoH Reddit: Make TW rewards based on the difference of the two scores