To put the computed return at the end time point of the interval, augment the diff vector with a missing-value indicator---
data.logret_GDAXI=[nan;diff(log(data.AdjClose_GDAXI))];
data.logret_GSPC =[nan; diff(log(data.AdjClose_GSPC)];
data.logret_N225 =[nan; diff(log(data.AdjClose_N225)];
Needing to extend the length of a diff series is a very common occurrence to keep sizes same for indexing or other similar purposes.