In law when seeking compensation you do not have to be guaranteed the sum you are claiming. There are many, many cases of this. One such example would be when people are injured at work where their employer is culpable and when the employee is forced to take long term sickness. In such circumstances the employee's can claim for potentially missed bonus' and overtime dispite such bonus' and overtime not being guaranteed. This is known in law as 'loss of chance'.
Only knowing the information that is in the public domain surrounding this matter, I would suggest that Gibson has a very strong claim to compensation through loss of chance. Factors such as there only being one point difference between the two clubs final standings would suggest that it is highly likely that Middlesbrough would have finished above Derby if Derby had not had the unfair advantage they gained through cheating the FFP rules.
Other factors such as how likely would Middlesbrough have been to have won the play off's and gain promotion had they had finished above Derby in 6th place would need to be carefully calculated.
The likely formula implemented to gain a figure for loss of chance would be to calculate a figure of what promotion is actually worth. There are many problems to this as it is almost impossible to gauge if Middlesbrough did win the 2018/19 play off's and gain promotion how likely would it be that Middlesbrough avoid relegation and how do you calculate how many seasons of Premiership payments can Gibson claim for. If you assume one season which is the worse case scenario then a calculation needs to be made on to how much is promotion to the Premier league for one season worth.
A calculation would need to be made of adding the income made from taking part and winning the playoff's plus the income of one season in the premier league plus the income of the subsequent three seasons of parachute payments. Once you have that figure which is usually estimated to be well in excess of £100m then you would need to divide thst figure by the percentage chance of Middlesbrough winning the play off's. From there you have to leave it to the lawyers and judiciary to cast their opinions. It is far from an exact science but there are many examples of this in compensation claims.