Hi, no need to speculate, you can read this from the GSM specs.
http://www.3gpp.org/ftp/Specs/archive/45_series/

See 45.001 for a "tutorial":

*start of quote*
6    Frequency hopping capability
The frequency hopping capability is optionally used by the network
operator on all or part of its network. The main advantage of this
feature is to provide diversity on one transmission link (especially to
increase the efficiency of coding and interleaving for slowly moving
mobile stations) and also to average the quality on all the
communications through interferers diversity. It is implemented on all
mobile stations.
The principle of slow frequency hopping is that every mobile transmits
its time slots according to a sequence of frequencies that it derives
from an algorithm. The frequency hopping occurs between time slots and,
therefore, a mobile station transmits (or receives) on a fixed frequency
during one time slot (» 577 µs) and then must hop before the time slot
on the next TDMA frame. Due to the time needed for monitoring other base
stations the time allowed for hopping is approximately 1 ms, according
to the receiver implementation. The receive and transmit frequencies are
always duplex frequencies.
The frequency hopping sequences are orthogonal inside one cell (i.e. no
collisions occur between communications of the same cell), and
independent from one cell to an homologue cell (i.e. using the same set
of RF channels, or cell allocation). The hopping sequence is derived by
the mobile from parameters broadcast at the channel assignment, namely,
the mobile allocation (set of frequencies on which to hop), the hopping
sequence number of the cell (which allows different sequences on
homologue cells) and the index offset (to distinguish the different
mobiles of the cell using the same mobile allocation). The non hopping
case is included in the algorithm as a special case. The different
parameters needed and the algorithm are specified in 3GPP TS 45.002.
*end of quote*

See 45.002 for the details (Subclause 5.4, 6.2, 6.2.2, 6.2.3, figures 4,
5 and 6 illustrate how GSM makes use of frequency agility):

*start of quote*
Specific parameters of the channel, defined in the channel assignment
message:
i)    MA: Mobile allocation of radio frequency channels, defines the set
of radio frequency channels to be used in the mobiles hopping sequence.
The MA contains N radio frequency channels, where 1 £ N £ 64.
    For COMPACT, the reduced MA (see 3GPP TS 44.060) shall be used for
a fixed amount of data blocks, see section 6.2.4.
ii)    MAIO: Mobile allocation index offset.(0 to N 1, 6 bits).
    For COMPACT, MAIO_2 shall be used for the data blocks using the
reduced MA.
iii)    HSN: Hopping sequence (generator) number (0 to 63, 6 bits).
*end of quote*

So, frequency hopping is possible on maximum 64 carriers, those can be
spread over the full frequency band (only the slot assigned for the
operator is limiting), if one operator had the licence for the DCS 1800
band, that would be 75 MHz.

The broadcast channel (BCCH) does not hop (the PBCCH may) but even in
that case it would be only one slot out of 8 where frequency hopping was
not supported (and that slot is not used as a traffic channel, no speech
etc.), the rest, 7 full rate channels or 14 half rate channels may use
frequency hopping.

The spec allows many options for the operator. Of course it is possible
to exclude frequency hopping totally if one so prefers. Very often
frequency hopping is excluded for the "BCCH carrier", in this case
frequency hopping is still applied on the rest of the frequencies
assigned for that particular cell (note that also the base station
normaly is frequency agile, more frequencies may be assigned and used
compared to the number of physical tansceivers at the base station).

For GSM, you do not have to believe on my views or the site referred
earlier, nor on the statements of a person who "knows" that CDMA makes
better use of frequency diversity, just go and check from the specs how
the system really works, the specs are available for everybody.

Sorry for long quotes.

Happy hopping (with or without GSM).