MTGA:

You need

2 Mythic
42 Rare
20 Uncommon
10 Common

Wildcards

Deck list in plain text

After MTGA's update, you can now import normal lists: copy the deck list below. Then open MTG Arena and, on Decks, click on 'Import Deck'.
(Arena language must be set to English):

4 Soul-Scar Mage
4 Symmetry Sage
4 Slickshot Show-Off
4 Dreadhorde Arcanist
2 Play with Fire
4 Wizard's Lightning
4 Flame of Anor
4 Sleight of Hand
4 Reckless Charge
2 Shove Aside
4 Expressive Iteration
1 Island
4 Spirebluff Canal
3 Fiery Islet
4 Riverglide Pathway
2 Den of the Bugbear
4 Steam Vents
1 Otawara, Soaring City
1 Sokenzan, Crucible of Defiance

2 Snapcaster Mage
1 Jegantha, the Wellspring
2 Spell Pierce
2 Surgical Extraction
2 Aether Gust
2 Shove Aside
2 Brotherhood's End
2 Pithing Needle

Paper:

Building Gruul Goblins should cost you about 279 € (Prices are calculated as mkm price trend ). Here are the missing cards you need to complete it:

4 Soul-Scar Mage
4 Symmetry Sage
4 Slickshot Show-Off
4 Dreadhorde Arcanist
2 Play with Fire
4 Wizard's Lightning
4 Flame of Anor
4 Sleight of Hand
4 Reckless Charge
2 Shove Aside
4 Expressive Iteration
4 Spirebluff Canal
3 Fiery Islet
4 Riverglide Pathway
2 Den of the Bugbear
4 Steam Vents
1 Otawara, Soaring City
1 Sokenzan, Crucible of Defiance

2 Snapcaster Mage
1 Jegantha, the Wellspring
2 Spell Pierce
2 Surgical Extraction
2 Aether Gust
2 Shove Aside
2 Brotherhood's End
2 Pithing Needle