As far as I know, the carryon restriction is implemented by boarding group. That is, if you board with group 5, then you don't get a carryon -- if you board earlier, you get to take a carryon. Generally, if two people are flying together and one has a higher boarding group, they are informally (but reliably) allowed to board together. So if your *G status puts you in Boarding Group 2 (or whatever) despite the BE ticket, then your companion will be able to walk up to the gate with you during BG2 and board with you -- and take a carryon bag.
I could be wrong about several parts of this -- I have no personal experience with it! I'm not 100% certain that *G status gets you a higher boarding group on a BE ticket, nor am I 100% certain that a BG5 passenger boarding with a companion in BG2 will be allowed to take a carryon.
And the checked bag question is totally separate, because that's a formal process at checkin. So United's official policy (whatever it is) is much more likely to be enforced there.
While GA's could certainly look up the fare basis for each passenger boarding, it is highly unlikely that they will. Thus, against the slim chance that one is required to check the bag and pay $55 vs. $30, I would take the $25 risk.