The Serial pins on the Arduino Mega 2560 are 5V. Those pins are directly connected to the ATmega2560 microcontroller which runs at 5V.
Reading a 3.3V input for the serial RX is almost never a problem. But it pushes the 5V TX level into the TTY and GPS. Some devices are 5V tolerant. Some devices are even 5V tolerant without any mention in the datasheet.
A very simple solution is for example to limit the current to 1mA with a resistor in the TX line.
5V - 3.3V / 1mA = 1700 Ω, that means a resistor of 1k8 to 10k.
The ATmega2560 can output 40mA per pin. That is a lot. When pushing 5V into a 3.3V chip via a TX pin, it is not 40mA, but it might be enough to damage some chips.
Many new sensors are not very tolerant. The 10k pullup to 5V on the Arduino Mega 2560 board can be enough to lift the voltage level of a 3.3V sensor via SCL and SDA to damage it.