When he said the M2 is a stopgap, he meant it won't be around as long:
Currently, the M chips are nearly year behind the A chips in 𝜇architecture: The Macs coming out now and for the rest of the year will be M2, but that's based on A15, while this November's upper-end iPhones will be based on the A16. Thus, according to Gurman, to close this gap, Apple will try to release the M3 as soon as possible. Thus the M2 won't be around very long.
I, however, am not sure if Apple wants the M-releases to catch up with the A-releases. Apple's strategy might instead be to have the iPhones lead the Macs in 𝜇architecture, while the Macs lead the iPhones in process level:
The M-chips are much bigger and more complex than the A-chips. So Apple's strategy might be for the first releases on the new 𝜇arch to be the simpler A-chips, and then gradually (as they increase their own understanding of the new 𝜇arch), release succesively larger and more complex chips. That's exactly what they did with the M1: first came the A14, then the M1, then the the M1 Pro/Max, and finally the M1 Ultra.
However, while the iPhones may lead the Macs in new 𝜇arch's, the converse may be true for process level, such that the first chips released on new process are the M-series (in particular, the higher-end M-series chips). Since initial volumes for a new process will be low, it makes sense to first use those new chips for lower-volume products and then, as production ramps up, move the highest-volume products (the iPhones) to the new process. That appears to be what they are doing now: The first devices to get the 3 nm chips will likely be the Macs released in early 2023. The iPhone (at least according to rumors) won't be getting 3 nm this year, so it will be Nov. 2023 at the earliest for it.