This may be one of the tricky concerns for GCASH. Because once your PIN is locked, you may not have access to the GCASH hotline temporarily too.
The best way to reset your PIN is to call the hotline and once asked for the concerned mobile number, input someone elses's GCASH number (namely a friend of yours or someone you know who has an active GCASH account), so you could push through the PIN Reset process.
Once the active number has been inputted in the IVR, you will then get the option for PIN RESET, and you will then speak to a customer service rep who will provide you with a temporary PIN which later on you need to further personalize.
And you are done!
I hope this helps,