Gifts are given freely from the One who has to the one who has not. The gifts do not provide any evidence as to what is inside a person’s heart.
Fruit, however, is produced from within and clearly shows what is inside a person’s heart.
So a believer’s spiritual maturity should not be determined by their gifts, but rather by their fruit.
A Christian who has the gift of prophesy, healing, or tongues is not necessarily a mature believer. The Corinthian church was operating heavily in the gifts, but they were all immature believers.
But look at the life of a Christian who is strong in love, is full of joy and peace, is patient, is kind, is good, is faithful, gentle, and can control himself–now there is a mature believer.
Notice that Galatians 5:22 does not say: “…the fruits of the Spirit are…”. It actually says: “…the fruit of the Spirit is…”. So even though there is a list of fruits, they are all to be considered one thing–they are a package deal. So you can not say something like: “I’m strong in all those fruits except self control and patience. But because I’m strong in 7 out of the 9 fruits, I’m a very mature Christian.” I don’t think you can say that–just like a chain is only as strong as its weakest link, you are only as mature as your weakest fruit.