PMIC Battery charger and power path
If you have a problem with GSM transmitter connected to the power managed by BQ25xxx IC, this is a different problem you are facing. 

RF transmitters consume a lot of current (~2 A pulses) when transmitting. Apparently your power supply path is not sufficient to eliminate drops (deeps) in power supply when the radio consumes a lot of current, and the charger IC has a very little to do with this. You need to beef-up the power net with lots of low-ESR capacitance.