Gasquet, of France, confirmed almost three weeks ago that he had tested positive for cocaine during last month's Miami event and is provisionally banned as he tries to clear his name.
"I support him. I'm certain that he's not taking anything," Nadal said after reaching the fourth round of the French Open.
"He's a good friend of mine and I discussed this with him last week and he's most certainly not taking cocaine. You know what the world is like today. You go to a party ... if you kiss a girl who's taken cocaine, anything can happen. This can destroy your career and this is most unfair," he said.
Nadal again criticized the World Anti-Doping Agency's so-called "whereabouts" rules, under which athletes must tell their national anti-doping authorities where they will be at a chosen hour between 6 a.m. and 11 p.m. each day for a three-month period.
If they change their plans they are obliged to inform the authorities of that change. Athletes who miss three doping tests in an 18-month period face possible suspensions.
Tennis players want more flexibility because their schedules change at the last minute if they are knocked out from a tournament in which they were expecting to do well.
"The International Tennis Federation should take measures," Nadal said. "I don't have the impression that it's good to put so much pressure on us. They harass us.
"If I lose tomorrow, I'll go back to Mallorca and who will know where I am if I have no access to the Internet? Now, if they knock at my door in Mallorca, they're going to give me a warning. It's happened to Carlos [Moya] before. They sent him a warning and this is most unfair," he said.
The ITF had no comment on the issue.