At first it seems politically peculiar: Gary Hymel, longtime right arm of House Speaker Thomas P. (Tip) O'Neill, is now working for Robert Keith Gray, longtime insider of the Republican establishment.

Really. And last night there was a party of 200 at Gary's place to prove it.

"Well," said a gruff Tip O'Neill, who attended, "they needed some brains and some talent over here."

Question: How does he feel losing his top aide to Gary, a man on the side of Ronald Reagan and the budget cuts O'Neill was fighting?

"I don't even know," O'Neill responded, "who Bob Gray is."

The host.

Gray gave the party at his Gray & Co. public relations office, named The Power House because it once generated just that. It's a huge renovated place in Georgetown with exposed brick walls, roll-top desks, clocks set in four different time zones, paintings by Dwight Eisenhower. And pictures of Richard Nixon.

"Oh, is he on the wall?" said Hymel. "I didn't notice. I walked by very quickly."

Hymel, widely regarded as one of the best-connected aides on the Hill, is now senior vice president of Gary & Co. His name is pronounced "HEEmel," but people always get it wrong. Why the move?

"Bob needed a Democrat," Hymel explained, "and you know, I had done it up there. Oh, have you met my daughter Judi? Judi is going to college next fall, joining her sister who is already in college. And their mother's in college, too. Now can you figure out why I did it?"

Hjmel, father of eight, will be making what he said is in "the six figures." On the Hill, his salary was in the $55,000 range.

"A pragmatist," an admirer remarked last night.

As for Gray, he gets a man who knows probably all the Democrats in Congress. "If you are a firm who is identified with the Republican inaugural," observed Chris Matthews, who is Hymel's replacement in O'Neill's office, "and you want to be able to talk to Democrats. . ." He looked around. "Let's face it," he continued, "this is alien territory for Democrats."

It was. Gray does have 23 of them in his 54-member firm, but Hymel is the first to enter the in-house hierarchy. Last night, all Hymel's Democratic friends made the place seem like an old Jimmy Carter party. The look was fresh-faced and prosperous, accented with a few mustaches. Everybody kept on saying how strange it was to be in the office of a man who was co-chairman of the Reagan inaugural, but you could tell they liked being there.

"We've got somebody to finance our reunion," said David Rubenstein, who used to work for Carter domestic policy adviser Stuart Eizenstat. Now both of them are well-paid Washington lawyers.

"I haven't seen this many Democrats having so much fun in a long time," said Gray. Then he went out to dinner.