Hi,
First read both datasheets.
especially:
V_IL, V_IH, V_OL, V_OH and the allowed (operational) input voltage range.
Then do a forum search for "logic level shift", "logic level converter" and simiala search.
This topic has been discussed many times.
Also have a look at the box "similar threads" below.
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Note: This is not a problem related to GSM or GPS, but it is a general problem with connecting ICs with different supply voltages.
Thus I don´t recommend to do a search with "GSM", "GPS" or "UART", because this just limits the hits.
Klaus