In order to accept GRAT tokens on your existing Stellar account you have to create a trusline. You can do this using any official Stellar wallet, trading client or exchange service like StellarX.com, StellarPort.io, Stellarterm.com or any other Stellar-compatible services listed here: https://www.stellar.org/lumens/wallets/
Below is the example with StellarTerm.com:
- Navigate to https://stellarterm.com/
- Click 'Login' button
- Click 'Login with key' button
- Enter your secret key (here you can find how to get you secret key from Gratzio wallet)
- Click 'Log in' button
- Click 'Accept and Continue'
- Click 'Accept assets' on the top bar menu
- Enter gratz.io In section 'Accept asset via anchor domain' in 'Anchor Domain' field
- Wait for a few seconds until it loads
- Click 'Accept GRAT'
- Now your account is ready to accept GRAT tokens.