[0.18] Items changes are huge on multiple items. Will have to review everything. A lot of the synergies are not as strong.

Note: This build does less damage than other Gideon builds, but has much better survivability. Enables you win poke fighting, and value from your passive and ultimate to setup easy kills and favourable teamfights.

Core Items

Three core items for the build give good Magical Power, Health, Ability Haste, and Mana. With shields to add to survivability while ulting in teamfights and some regen to sustain lane phase and mid game.

Crest: Soulbearer

Correct stats, and an active cast shield scaled off Mana. While also gaining Magical Lifesteal and movement speed.

Orb of Growth (Orb of Enlightenment)

Correct stats, with even better numbers when it evolves. On ability cast restore 3% missing HP is huge for lane phase and will allow you to win repeated short trades.

Truesilver Bracelet

Correct stats, and gives a shield that scales off HP when you ult, immunity to CC, and explodes for extra damage. Enables better survivability when ulting into a teamfight.

Standard Item Choices

Third Item

Oathkeeper

Empowered basic attack after using an ability, scales with Magical Power. Increases full non-ult combo damage by weaving in auto attacks between abilities. Plus it gives the correct stats, Magical Power, Health, and Ability Haste.

Azure Core

With Orb of Enlightenment (lvl 18), and Azure 400 stacks, you will have 2445 max mana. Meaning at full build this item will give 146 magical power, and Soulbearer's Shield will be 611 max health.
The downside is due to needing to get the 400 stacks (5 per any kill), the power spike for this comes later than simply finishing Oathkeeper and having empowered autos for combos. Consider buying the first part of the item, as it will already have the passive for stacking and scaling. Finish the item if it makes sense to do so.

Tainted Scepter [Situational]

Anti Heal option, if the other team has high amount of healing on it this becomes valuable. Malice gives some extra damage, but Oathkeeper's empowered autos are better. Stats are very similar, but Oathkeeper is slightly better.

Megacosm [Situational]

Max HP% Damage, if the other team is very beefy, the value of HP% damage might be worth it. Stats are similar to the other two, except it swaps the health for mana. Soulbearer's shield does scale off Mana, so you still do get some added survivability, but it is not as valuable as the max health.
With less HP, World Breaker 4th, and Lifebinder 5th both lose some value.

Fourth Item

World Breaker

Magical Power, Health, rare source of Tenacity stat. Stacking damage on dealing magical damage, good value from combo and ult. Gain Magical Power based on max health, which this build is stacking a lot of!

Fifth Item

Lifebinder

Good Magical Power, and Max Health. Plus Magical Lifesteal for healing the large health pool outside of using Soulbearer. Additionally, gives increased Ability Haste that scales off Max Health, and Magical Power - the two stats we have been building with every other item.

Oblivion Crown

More expensive, but huge Magical Power increase. Contains large Magical Power, and gain more based on how much Magical Power you have. Plus, 30s cooldown of 10% max health damage. A lot of extra damage, especially if there is an enemy with a lot of HP.

Caustica

Magical Power, and ignore 40% of magical armour. Useful if some enemies are stacking Magical Armour.

Items Not Worth

Some items can definitely work on Gideon, but with this build end up not being the best choice.

Timewarp

At full build Lifebinder gives better cooldown reduction due to the Ability Haste scaling with Health and Magical Power, which this build has a lot of.

Astral Catalyst

At full build Lifebinder gives better ultimate cooldown reduction due to the Ability Haste scaling with Health and Magical Power, which this build has a lot of.