US involvement can be summarised as vague approval of the use of force, surprise at the scale of the use of force, and acceptance of it as necessary, and cooperation after the events.

The US had, over a week before 18th May (on 9th May), told the Korean government that they did not oppose the use of the army to help the police maintain law and order. This was communicated by the US Ambassador (Gleysteen) to President Chun, and also direction from the US State Department. On 16th May, the US commander-in-chief of the Korea-US Combined Forces released the 20th Division from his operational control to be freely used by the Korean government in Gwangju.

There is no indication that the US expected the level of violence that occurred, but clearly some violence was expected when thousands of troops were used against rioters. When the scale of events became clear, the US response was one of acceptance, and after-the-fact approval. As Ambassador Gleysteen reported to the US on what he told the Korean government on 23rd May,

I made clear our unhappiness with the events of May did not related to measures to control rioting, which were necessary, but to the accompanying political crackdown which had exacerbated existing problems and helped bring about the Kwangju crisis.

After the events, the US cooperated with the Korean government efforts to control the news about Gwangju (presenting the events as the suppression of a communist uprising rather than the suppression of a pro-democracy movement, and downplaying the scale of the incident).

Whether the US could have prevented the us of military force isn't clear. It is quite possible that they could have, since US disapproval of the government decision to deploy the army to suppress the protests in the June Democracy Movement in 1987, communicated by the US Ambassador and directly from the US, led to the reversal of mobilisation orders within hours (before troops were actually deployed). This led to the end of the military dictatorship in Korea, when the government bowed to popular pressure and accepted a transition to democracy. Since US pressure was able to prevent what could easily have been Gwangju times 10, it is possible that US pressure might have prevented the Gwangju Massacre 7 years earlier.

The US had been much more directly involved with the suppression of the Jeju Uprising in 1948 (a much deadlier affair, with about 10% of the population of Jeju being killed, and over 10% fleeing to Japan as refugees). The suppression began when South Korea was under the control of the US military government (USAMGIK), and the security forces on Jeju were under US command. Reportedly, US forces on Jeju (which were few in number) did not directly take part in the killings, but observed them without interference. The US did not have this level of control over events during Gwangju, and didn't have troops on the ground in Gwangju watching the events unfold. As said in the beginning, it was a matter of a vaguely-expressed approval of the use of military force if necessary, and acceptance of the events afterwards.

Reference:

The Gleysteen quote is from the cable in https://kr.usembassy.gov/wp-content/uploads/sites/75/014-87df.pdf

Other relevant communication by the US Embassy are available at https://kr.usembassy.gov/our-relationship/official-reports/may-18-documents/