I have to correct you on this, Umberto. The ERJ-145 actually has stairs as options, as it either has internal airstairs or it has the sliding door. Here's an American Eagle ERJ-145 with the sliding door.
No correction was needed. GSX is always based on some kind of existing airplane model in FSX. In this case, the ERJ-145 that GSX supports is the Feelthere, which comes only with an internal airstair, it's not like the 737 NGX that has airstairs as an option.
That's why GSX correctly assumes THAT ERJ-145 will always provide its own stair, and it won't show the GSX front stair.