There is not working hook for multiplying crop gather:

        if (player != null)
        {
            player.GiveItem(item, BaseEntity.GiveItemReason.PickedUp);
            Interface.CallHook("OnGrowableGathered", this, item, player);
            return;
        }​

For working must be:

        if (player != null)
        {
            Interface.CallHook("OnGrowableGathered", this, item, player);
            player.GiveItem(item, BaseEntity.GiveItemReason.PickedUp);
            return;
        }

With hook OnGrowableGather it is impossible to change the number of gathered resources.

Is OnGrowableGather not enough for you? OnGrowableGather(GrowableEntity, BasePlayer).

The problem is that the original method where OnCropGather was ended up being split in Rust (and PlantEntity replaced with GrowableEntity. The issue with this is that Item was no longer available in the original method, so it had to be moved or arg dropped. If moved, it was no longer possible to cancel the original behavior, but the original location no longer has the Item object available.

With Post hooks (OnACTIONed, they are generally placed after the action has already happened, not before like Pre hooks.

I tried to change the number of gathering resources, but nothing good happened. Some values there ​​are read only. Example: this.currentStage.resources only GET.