It is possible to use SAP Router String, e.g.
"/H/" in the RfcConnect command line? 
there any documentation on the RfcConnect command?

Yes, it is possible to specify the SAP Router string in RfcConnect, e.g. R3NAME=/H/sapgate1/S/3297/LOI 
where LOI is the R/3 system name.

Please see the SAP documentation on the RfcOpenEx function for details (file saprfc.h). GuiXT will add user, password, client and codepage to the RfcConnect string provided in GuiXT profile, and then perform the RfcOpenEx function.