The datasheet states 6 mA as the maximum current that can be sunk or sourced while keeping the voltage 0V or 3.3V, respectively for most pins ("all other LCVMOS pins" in section 5.7 "DC electrical characteristics). However, somse GPIO pins have a 4 mA limit, namely:
ECAP0_IN_PWM0_OUT,UART0_CTSn,UART0_RTSn,UART0_RXD,UART0_TXD,UART1_CTSn,UART1_RTSn,UART1_RXD,UART1_TXD,I2C0_SDA,I2C0_SCL,XDMA_EVENT_INTR0,XDMA_EVENT_INTR1,WARMRSTn,EXTINTn,TMS,TDO,USB0_DRVVBUS,USB1_DRVVBUS