As it stands, there is no way to add customization before the effect has applied. The only alternative is CanGameplayEffectApply, however the spec received as param is const and I am not supposed to use it.
bool CanGameplayEffectApply(const FActiveGameplayEffectsContainer& ActiveGEContainer, const FGameplayEffectSpec& GESpec) const override;
void OnGameplayEffectExecuted(FActiveGameplayEffectsContainer& ActiveGEContainer, FGameplayEffectSpec& GESpec, FPredictionKey& PredictionKey) const override;
void OnGameplayEffectApplied(FActiveGameplayEffectsContainer& ActiveGEContainer, FGameplayEffectSpec& GESpec, FPredictionKey& PredictionKey) const override;
bool OnActiveGameplayEffectAdded(FActiveGameplayEffectsContainer& Container, FActiveGameplayEffect& GE) const override;