Keep HODLing is an Android app that helps you pile up CryptoCurrencies using Dollar Cost Averaging.
You can buy Bitcoin, Litecoin or Ethereum using US Dollars and Euro. You can also buy Bitcoin using GB Pounds.
The app interacts with the GDAX Exchange using APIs. To use this app you need a verified GDAX account. Open GDAX.
Once your account is ready go to the API Settings page, and generate a new API Key and Secret pair. When creating the API Key select at least the View and Trade permissions. Also select Withdraw with 2FA bypass to allow the app to move bitcoins to your private wallet.
The app was created because I wanted something like this, but I do not have access to a Windows machine and most important I do not want to have a PC turned on 24/7.
So I took the concept and made an app around that. I hope this helps you too.