12.5kHz or 25kHz refers to the channel spacing for "narrow" and "wide" channels respectively. This is the channel spacing for each type, NOT the bandwidth of each. The occupied bandwidth (OBW) of each is slightly narrower. Channel spacing and OBW are not the same thing.
GMRS has historically been "wide" on the primaries AND on the interstitials, long before the bubble pack invasion began. GMRS can also operate "narrow" on the primaries and the interstitials. Take your pick.
OTOH, FRS (14-channels bubble packs) has been narrow from day one. The 22-channel GMRS/FRS bubble packs are narrow only on all channels, so they already are using narrow on the GMRS primaries.
For GMRS, I recommend running narrow only on the interstitials. On the GMRS primaries, I recommend running wide or narrow, depending on your preference and equipment capabilities. I for one are running narrow on all interstitials and GMRS primaries, and I'm using commercial-grade equipment instead of bubble packs. My UHF commercial radios specifically have FCC Part 95 Type Acceptance in addition to Part 90, so the equipment is legal on GMRS. (They're KENWOOD radios.)
I've had one of my GMRS repeaters get hit by FRS traffic on the upper FRS channels spaced 12.5kHz from the repeater input. The repeater is OLD (Kenwood TKR-820) and is "wide" bandwidth. My other GMRS repeater is wide/narrow capable and I'm using it in narrow mode to help prevent FRS from clobbering the input from 12.5kHz away on each side. It works.
A few years ago I finalyl stepped up to the plate and I'm running narrow mode on all GMRS primaries and on the interstitials. I found all of my adjacent channel splatter problems from bubble packs in my neighborhood wend away as soon as I switched everything to narrow mode. I also decided to go with the Part 90 flow of things where Part 90 services are in the middle of transitioning to narrower bandwidth and older "wide" equipment will become hard to get in a couple of years.
Even though I'm using good commercial-grade equipment, I'm operating narrow bandwidth like the 22-channel bubble packs already are. All of my adjacent channel splatter problems went away as soon as I made the switch.