Can I use MCU's internal oscillator when communicating with GPS and cellular modules?
Asynchronous serial communications places demands on the accuracy of the clock. Some internal calibrated RC oscillators are acceptable, some are not, and some are okay  over a limited temperature/Vcc range and/or if you don't have additional errors due to divider resolution (typically at higher speeds). 

The receiving party starts timing the data based on when it detects a starting edge. That detection will be degraded a bit if the cables are long and by the number of times it samples within a given bit. Since the framing typically takes place on a per-byte basis, the error accumulates within the byte plus start bit, stop bit partity etc. which is typically about 10 bits per byte. So a 5% error means the receiver is looking at the data 1/2 bit off, which is right at the edge if everything else, including the receiver clock frequency is perfect. 

If the other party is using a crystal controlled clock, a worst-case error of ~4% is probably (barely) acceptable. Usually 2% is used as a limit, if you don't have control of the other side. 

An MCU with a 1% worst-case initial error (large print number) may have unacceptable or barely acceptable accuracy when all factors such as voltage coefficient and temperature coefficient are considered. For example, a typical spec from a "midrange" Microchip part 



If your only need for serial communication is in an office/shop environment for configuration or diagnostics, the above is more than good enough for communications with a PC that is guaranteed to have a proper crystal controlled clock. If you expect it to work in a car, under all conditions, it's not guaranteed to be good enough. 

If that is your only requirement for an accurate clock, I would suggest considering a ceramic resonator. It's not necessarily cheaper, but results in a lower parts count and is more rugged compared to a crystal oscillator and is typically more than good enough (0.5% is a typical spec) in accuracy for asynchronous serial communications.