In the person to person payments, as the name suggest, two individuals transact with each other. There are various payment methods available in the online world. One can use Bank transfer like NEFT or RTGS, Visa Card Transfer, Paypal or IMPS (Interbank Mobile Payment System). Also, in some countries Mobile Wallets allow person to person money transfer, like M-Pesa of Kenya.
For cross border payments, one can use Western Union or Banks can have arrangements with other banks through SWIFT Worker's remittance which follows a ISO20022 message format and allows person to person mobile money transfers.
From a Payment Gateway provider's perspective; P2P is not of much interest unless, it is also a provider of Mobile Wallet and need to provide interoperability. In India, NPCI acts as the ACH for IMPS and semi-closed wallets are not inter-operable.
Payment Gateways can monetize the P2P payments by providing gateway services to Mobile Wallets providers for:
Western Union Integration
Visa Card P2P Integration
SWIFT Workers Remittance Integration
IMPS integration (In this case a schedule bank's license is leveraged and the bank assigns a MMID (Mobile Money ID) to the Wallet).
The payment gateway provider will provide a uniform API and a unified business process and reconciliation procedures to the mobile wallet operator.