Either way, peeking through the list, seems to me names which end with a vowel are usually feminine. However, there's a caveat: names end up shortened, and over time even those get shortened.
'Brit', 'Britney' for 'Brittany'.
'Gerry' or 'Geri' or 'Ger' for 'Gareth' (Dutch variant: 'Gerrit'),
'Pat', 'Patty' for 'Patrick', 'Patricia'.
'Pen', 'Penny' for 'Penelope'
'Ash' for 'Ashley'.
'Tash' or 'Tasha' for 'Natasha', 'Nata(s)cha' (a variant of Nathan/Nathaniel?)
'Toni', 'Tony', or 'Ton' for 'Antonia', 'Anton', or 'Antonius'.
Skimming through the quick examples I came up with, one thing stands out: if it ends with an 'a', feminine. I believe this stems from Latin? It also appears Brittany is a very British example, whereas say 'Tony' stems from a Catholic background. People with that background get very traditional names, but they're always practically shortened. Annoyingly, you'd have to mention your complete name on say a form, but otherwise you get a wider choice since you have like three names, and they're relatively long so more variety to pick from.