There aren't any "sub-channels" in FRS/GMRS radios. There are frequencies and there are "interferance eliminator" codes.

The frequencies are as follows:

These seven frequencies (MHz) (channels) are shared between FRS & GMRS. An FRS-approved radio can only broadcast at max 0.5 Watt. Most hybrid radios broadcast on these frequencies at a higher power, which is why use of hybrid radios require a GMRS license:
1 462.5625
2 462.5875
3 462.6125
4 462.6375
5 462.6625
6 462.6875
7 462.7125

The next seven frequencies (channels) are for FRS use only. FRS radios can only broadcast at 0.5 Watts max. GMRS radios may not broadcast on these frequencies. Hybrid radios tend to broadcast on these frequencies at only 0.5 Watt.
8 467.5625
9 467.5875
10 467.6125
11 467.6350
12 467.6625
13 467.6875
14 467.7125

The following seven frequencies (channels) are for GMRS use only. FRS-approved radios will not broadcast on these frequencies. Hybrid radios can broadcast on these frequencies, which is also why a GMRS license is required.
15 462.5500
16 462.5750
17 462.6000
18 462.6250
19 462.6500
20 462.6750
21 462.7000
22 462.7250

Some feel that a hybrid FRS/GMRS radio CAN be used without a GMRS license if ONLY channels 8-14 are used for broadcasting. Last I had heard the FCC hasn't given a clear interpretation on this usage.

FRS radios tend to have 14 frequencies (channels) and nonremovable antennas.

GMRS radios tend to have 14 frequencies (channels). The first seven are the same as those on FRS radios. "Real" GMRS radios tend to have removable antennas.

Hybrid GMRS/FRS radios tend to have 21 frequencies (channels). Channels 1-7 and 15-21 tend to broadcast at higher power (1-2 Watts), while channels 14-20 tend to broadcast at 0.5 Watts or less. To my knowledge, all hybrid radios have non-removable antennas.

The interference eliminator codes work differently. A radio will only respond (play) if the broadcast also includes a particular eliminator code frequency. The idea is that if you are using frequency 1 (462.5625 MHz) with code 5. Your radio will only "play" broadcasts that are on frequency 1 with code 5. Other folks can use frequency 1 too, and if using a different code then you won't have to hear them talking. The problem is that you are both using frequency 1, so if you both try to broadcast at the same time then you will be interfering with each other (crosstalk). Some radios show a symbol on the display when the selected frequency is in use, even if on a different code, but few users look at the display before broadcasting. Less fancy radios require the user to press a monitor button to listen the the "raw" frequency and determine if anyone else is broadcasting attempting to broadcast. Most users don't do that either.

Many users recommend simply not using the codes at all (sometimes this can be done by setting the radio to code 0). This means you'll hear every broadcast on the frequency you're uisng, but the benefit is that there is little chance of broadcasting over another user (crosstalk).

Another thing to keep in mind when selecting a radio is that reception distance tends to be a factor of antenna height much more than of broadcast power (Wattage). GMRS users who want longer distance but cannot access a repeater will tend to raise a remote antenna high up in a tree or on top of a building and then connect to that antenna. Picture throwing a rope over a nearby tree and pulling an antenna high up a tree to act as a "base station". FRS and Hybrid users don't have that option, since their antennas are not removable.