Why DC block is used for GPS receiver tested using a simulator via an active antenna?
I would have to see a diagram of the test setup you're referring to to be sure, but I presume this is to block DC bias that the GPS receiver provides to power an active antenna so that it doesn't damage parts of the test setup that aren't designed to handle this, or damage the receiver by drawing too much current.  It's common for GPS receivers that use external antennas to supply anywhere between 3.3v to 12v out the antenna connector with a bias tee.  This can be used to power an LNA integrated with the antenna to compensate for cable losses.  12 volts into 50 ohms is almost 3 watts, which could be a lot more than what the attenuator is designed to handle, and it could be a lot more than the receiver is designed to provide.  An active antenna with integrated amplifier will have a built-in bias tee to extract this bias voltage, so in effect the receiver will see a much higher impedance than 50 ohms at DC.