Answer to your first question: As far as I know, the USRP device that you use has a maximum gain of 30 dB. So, I recommend you decreasing the gain a little.
Second one: As you may know, the USRP devices firstly amplify and downconvert the received signal, then sample it. The two first RF stages have a part called "DC choke (maybe RF coupling) " which prevents the DC component of your received signal from flowing through the baseband processing stages (in addition DC signals are not desired in RF circuits, and actually harmful). As you expected, in the ideal case, the peak on the origin should not be seen since the signal is collected with zero mean. However, if your signal contains long non-alternating sequences, RF choke can be penetrated by the DC component and your baseband signal has a DC level as well (DC leakage). Depending on the length of the non-alternating part of your signal, a fading on the DC level may be observed. This is called "DC drop", causing a spectral leakage in the vicinity of origin.