Take a look at this excerpt from the Part 95 rules governing GMRS. It tells you specifically what frequencies can be used for transmitting, based on what device is doing the transmitting.
Your repeater is limited to transmitting on one of the 8 frequencies listed under subsection A. To my knowledge, there is no rule that says the offset must be 5 Mhz. But, the biggest offset you can acheive with rules-accepted GMRS frequencies is (if my math is correct) 5.175 Mhz. Using an offset greater than that will put you out-of-band, either on your repeater's transmit frequency or its receive frequency, (or both) and therefore in violation of FCC rules.
§ 95.1763 GMRS channels.
The GMRS is allotted 30 channels—16 main channels and 14 interstitial channels. GMRS stations may transmit on any of the channels as indicated below.
(a) 462 MHz main channels. Only mobile, hand-held portable, repeater, base and fixed stations may transmit on these 8 channels. The channel center frequencies are: 462.5500, 462.5750, 462.6000, 462.6250, 462.6500, 462.6750, 462.7000, and 462.7250 MHz.
(b) 462 MHz interstitial channels. Only mobile, hand-held portable and base stations may transmit on these 7 channels. The channel center frequencies are: 462.5625, 462.5875, 462.6125, 462.6375, 462.6625, 462.6875, and 462.7125 MHz.
(c) 467 MHz main channels. Only mobile, hand-held portable, control and fixed stations may transmit on these 8 channels. Mobile, hand-held portable and control stations may transmit on these channels only when communicating through a repeater station or making brief test transmissions in accordance with § 95.319(c). The channel center frequencies are: 467.5500, 467.5750, 467.6000, 467.6250, 467.6500, 467.6750, 467.7000, and 467.7250 MHz.
(d) 467 MHz interstitial channels. Only hand-held portable units may transmit on these 7 channels. The channel center frequencies are: 467.5625, 467.5875, 467.6125, 467.6375, 467.6625, 467.6875, and 467.7125 MHz.
Before you make any conclusions, try separating the radios like they would normally be used, like a hundred yards or something, and see how they work. Radios aren't really designed to work that well that close together.
