The last complete election cycle (2004):
Rank: 1
Organization: Microsoft Corp
Amount: $3,310,270
Democrats: 62%
Republicans: 38%
And his largest individual contributions went to various Republican parties.
http://www.opensecrets.org/indivs/search.asp?Order=A&txtName=gates%2C&txtState=WA&txtZip=&txtEmploy=&txtCand=&txt2006=&txt2004=Y&txt2002=&txt2000=&txt1998=&txt1996=&txt1994=&txt1992=&txt1990=&txtSoft=N
Republican Party of Florida
Republican Federal Cmte of Pennsylvania
Republican Party of Ohio
Republican State Cmte of Michigan
Americans for a Republican Majority
Keep Our Majority PAC
Missouri Republican State Cmte
Arizona Republican Party
Republican Party of Wisconsin
Republican Party of Minnesota
Republican Party of Iowa
Republican Party of Oregon
I don't see anything going to any DNC groups.