Unfortunately it's not possible today to obtain the "ATM" card online. The ATM card, for reference, is called the PowerPay+ and you can find more information about it here:
It's actually more beneficial, if you can, to obtain it from one of our GCash outlets because you can do the KYC (Know Your Customer) process at the same time. It allows you to perform many of the additional GCash transactions and services we have available.
If you do not yet have GCash you can begin the initial enrollment through dialing *143# on your handset and then selecting the GCash option.
I trust this information helps.