They can be nicely separated on a reverse-phase C18 column, ACN-H2O as your mobile phase, and 0.1 TFA in your ACN to help give a better peak shape because gallotannins are quite acidic.
HPLC is just fine for the analysis of gallotannins, esp in fruits. Detection is another matter. They have a primary sugar unit, so Mass Spec is often used (LC/MS) for analysis. They also vary in Mw (low to high) so require excellent sample prep and extraction, the use of a column with a pore size large enough to retain them and a LC/MS method which can detect low and high Mw forms.
*If you run a keyword search on the web, you find plenty of application notes and papers.