If GameStop's market value is $12 billion, and the offer is valued at $55 billion, how can it be "split evenly between cash and GameStop common stock"? If the combined value of the 2 companies is $67 billion, then GameStop can offer at most 18% in stock (if all the GameStop holders are willing to chuck their stock into the offer), and 82% would have to be cash - from some other source.
OK, after reflection and calculation, I can sort of see how this adds up - if the current combined value is reckoned to be 12+46=58, then giving the current eBay shareholders nearly $28 billion means they'd expect their remaining shares to be worth at least 46-28=18 billion, of the combined company now worth 12+46-28=30 billion. If GameStop holders reckon the value of their holding shouldn't go down, then that would leave exactly $18 billion for eBay shareholders to own of the combined company (so 60% of it goes to eBay owners, and 40% to GameStop holders).
But ... on further reflection, you still can't call that deal "split evenly". For that, it'd have to be "$27.75 billion in cash, and what you end up with is also worth $27.75 billion". So that would have to be $27.75 billion of what GameStop is claiming a combined value of 12+27.75=$39.75 bn. That would be 70% of the combined company.
But this implies the eBay owners would have to reckon GameStop executives - whose company nearly went under - would run eBay better than the current eBay executives. With GameStop, a struggling company (their revenue is decreasing), attached.