Dont confuse the two.

GSM is a standard frequency network that most 2g/2.5g phones use, as
compared to PCN, which is a slightly different standard. 3g networks tend to
be UTMS or EDGE.
GPRS is a method of accessing a network, that allows an almost continuous
connection rather than requesting a connection. GPRS allows for data calls
at the same time as voice, and is therefore used predominantly for data.
When referring to GPRS, most retail networks (ie o2, t-mobile, vodaphone
etc) mean a data connection to a GSM network, however GPRS is also used for
3g data transfers, just as a higher bitrate.

Most people assume that GSM means a low bitrate connection, GPRS = faster
and 3g = even faster. Thats not the case. Read around the subject
technically and dont go by telecoms leaflets because their marketing tends
to confuse the issue.

(GPRS) General Packet Radio Service

http://www.google.com/search?hl=en&q=define:GPRS

and

(GSM) Global System for Mobile communications

http://www.google.com/search?hl=en&oi=defmore&q=define:GSM