Apart from Bahrain and Oman, where authorities cracked down on pro-reform protests in 2011, Gulf Cooperation Council states have been spared much of the unrest brought in by the Arab Spring uprisings that rocked the Middle East.
The GCC also comprises Kuwait, Qatar, Saudi Arabia, and the United Arab Emirates.
Many activists in the wealthy Gulf states, most of which ban political parties and demonstrations, have resorted to social media platforms such as Twitter, Facebook and YouTube to advocate human rights.
The New York-based watchdog made its latest call for GCC reform as it launched an interactive website with the profiles of 140 prominent Gulf activists -- reflecting Twitter's 140-character limit -- who have been arrested, tried and sentenced for voicing their opinions online over the past six years.
They include prominent Bahraini activist Nabeel Rajab, who is on trial on charges of spreading false information and posting online insults, and Saudi activist Waleed Abulkhair who is serving a 15-year jail sentence.
Also among those listed is Emirati lawyer Mohammed al-Roken, who was jailed in 2013 with 69 people for 15 years after being convicted of plotting to overthrow the regime.
"The Gulf states have engaged in a systematic and well-funded assault on free speech to subvert the potentially transformative impact of social media and internet technology," said HRW's Middle East director Sarah Leah Whitson.
"Instead of hauling off their peaceful online critics to jail, Gulf governments should expand debate among members of society and carry out the much-needed reforms that many of these activists have demanded for years," she said.
Hundreds of dissidents, from political activists to journalists and lawyers, have been imprisoned in the Gulf after "unfair trials", HRW said.
- Intrusion software -
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"GCC rulers' sweeping campaigns against activists and political dissidents have included threats, intimidation, investigations, prosecution, detention, torture, and withdrawal of citizenship," it added.
GCC governments also used surveillance technology bought from Western and Israeli companies to track and monitor their citizens' online activity, it said.
All GCC governments except for Kuwait have used intrusion software, said HRW, citing evidence from Toronto-based research group Citizen Lab.
"This software can enable a government to access emails, text messages, call histories, contact lists, files, and potentially passwords, and can allow authorities to turn on a phone or laptop's camera and microphone to take pictures or record video and conversations without the owner's knowledge," the rights group said.
In August, Apple released a security update to iPhone owners after a sophisticated attack on Emirati human rights advocate Ahmed Mansoor exposed vulnerabilities targeted by cyber arms dealers.
Although the cyber attack on Mansoor was not linked to a specific government, Citizen Lab had said indicators pointed to the UAE.
The UAE authorities never commented.
GCC states had also toughened laws since 2011 to punish "speech they deem 'criminal', particularly online and via social media networks", said HRW.
New "repressive" laws were enforced to punish "political dissidents and activists who criticise not only their own leaders but those of other GCC states and their policies".
These laws branded government critics as "terrorists" and granted authorities the right to strip protesters and dissidents of their nationality, HRW said.
Revoking citizenship has been carried out mostly by Bahrain, and to a lesser extent by Kuwait and the UAE.
Authorities in Bahrain have stripped at least 261 people of their citizenship since 2012, according to the Bahrain Centre for Human Rights, including the country's Shiite spiritual leader Sheikh Isa Qassim.
Whitson believes Gulf states "are seriously mistaken if they think they can indefinitely block Gulf citizens from using social and other media to push for positive reforms".