Fusion GPS is an opposition research/espionage firm, mostly used to attack Republicans and conservative groups. They aren't lawyers (so no privilege), they aren't even journalists (anymore). They are investigators hired privately to dredge up sleaze (or as is contended by some make it up).
Why aren't Republicans more aggressive in subpoenaing Fusion GPS? It is not just Trump. Fusion GPS was hired by Planned Parenthood to go after the people who did the undercover videos. Here's a bombshell that PJ Media reported: Fusion GPS allegedly also worked for Vladimir Putin.
EBL: Fusion GPS was in on this
Hot Air: Fusion GPS, Manafort, and FBI
AoSHQ: Media now covering up Fusion GPS
Powerline: Fusion GPS and Stand by your Awan
Circa: Fusion GPS and the Don Jr. meeting and FBI leaking
The Federalist: Americans need to know more about Fusion GPS
Daily Caller: Why's Fusion GPS getting a pass?, Highly paid smear merchants, pleading the 5th
Wombat: American Thinker: Wasserman-Schultz Subverted Democracy, Shark Tank: Fired Dem House IT Aide Has Connections To “Hezbollah-Like” Character, Mark Steyn: The Democrats’ Bet Pays Off, also, Light My Fire
Instapundit: Fusion GPS and its Russian Connections, Roger Simon: Mueller looks ridiculous, Meanwhile, this looks suspicious, Russians playing both sides, Democrats really want to divert attention, nothing to hide?, shopping, let's investigate this fully, and manufactured fake news