Your are saying this because his not an England player, if he was an English player you wont say 100million is small for such a talented striker like Viktor, how much did Manchester city bought Gealish from Aston villa was it not 100m, so Viktor worth 100m too, any club that can pay should pay and get him, I know Arsenal won't pay such amount for him, so Manchester united is at advantage because Amorim can convinced him to join Manchester united with him as his former coach, Manchester united has a chance to sign him than Arsenal. But Manchester united won't spend such amount on him, they will negotiate the best price.
I think the criteria for getting Gyökeres to sign goes beyond player and ex manager relationship. Manchester United are very unlikely to cough out that huge amount to sign Gyökeres at the moment and if any team will be ready to pay such an amount it will probably be Arsenal over Manchester United, therefore Arsenal have a bigger advantage in regards to this deal.