You can find Sun Cellular load products under Smart in Buy Load.
STEP 1: Launch the GCash App
STEP 2: Tap Buy Load
STEP 3: Select Smart as the telco, input mobile number of recipient, tap Next
TIP: If your recipient is in your phone’s contacts, tap the phonebook icon on the right and select the contact you wish to buy load for. This will automatically enter their mobile number in-app.
STEP 4: Tap Yes, Proceed
STEP 5: Input the amount you want to purchase or view the Buy Load offers
NOTE: All load products available for Sun subscribers will be labeled with Sun (i.e. Sun CTC 50)