I don't think UTF8 is supported by GSM. From http://en.wikipedia.org/wiki/Short_Message_Service
Short messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet, the 8-bit data alphabet, and the 16-bit UCS-2 alphabet
and from http://en.wikipedia.org/wiki/GSM_03.40
The messages in Chinese, Korean or Japanese languages must be encoded using the UTF-16 character encoding
The Data Coding Scheme (TP-DCS) field contains primarily information about message encoding. GSM recognizes only 2 encodings for text messages and 1 encoding for binary messages:
GSM 7 bit default alphabet (which includes using of National language shift tables as well)
UCS-2
8 bit data
In the same paragraph they tell that there is a new national-based encoding (the National language shift table) that was introduced in 2012. Still this isn't UTF-8.