The Nazi's killed other groups besides Jews, but the term Holocaust refers to the killing of 6 million Jews by the Nazis and their collaborators:

The Holocaust was the systematic, bureaucratic, state-sponsored persecution and murder of approximately six million Jews by the Nazi regime and its collaborators. During the era of the Holocaust, German authorities also targeted other groups because of their perceived “racial inferiority”: Roma (Gypsies), the disabled, and some of the Slavic peoples (Poles, Russians, and others). Other groups were persecuted on political, ideological, and behavioral grounds, among them Communists, Socialists, Jehovah’s Witnesses, and homosexuals.

As to the Nazi's use of gas:

Auschwitz-Birkenau also contained the facilities for a killing center. It played a central role in the German plan to kill the Jews of Europe. During the summer and autumn of 1941, Zyklon B gas was introduced into the German concentration camp system as a means for murder. At Auschwitz I, in September, the SS first tested Zyklon B as an instrument of mass murder. The "success" of these experiments led to the adoption of Zyklon B for all the gas chambers at the Auschwitz complex.

Near Birkenau, the SS initially converted two farmhouses for use as gas chambers. “Provisional” gas chamber I went into operation in January 1942 and was later dismantled. Provisional gas chamber II operated from June 1942 through the fall of 1944. The SS judged these facilities to be inadequate for the scale of gassing they planned at Auschwitz-Birkenau.

Four large crematorium buildings were constructed between March and June 1943. Each had three components: a disrobing area, a large gas chamber, and crematorium ovens. The SS continued gassing operations at Auschwitz-Birkenau until November 1944.

Concentration camps were supposed to be forced labor camps (although many were killed there) while the "killing centers" were death factories.

Concentration camps served primarily as detention and labor centers, as well as sites for the murder of smaller, targeted groups of individuals. Killing centers, on the other hand, were essentially "death factories." German SS and police murdered nearly 2,700,000 Jews in the killing centers either by asphyxiation with poison gas or by shooting.

And yes, the Nazis killed German Jews as well as Jews from other countries:

The victims of the Operation Reinhard killing centers included Polish, German, Austrian, Dutch, French, Czech, and Slovak Jews as well as Roma (Gypsies), Soviet prisoners of war, and Poles. The SS and police killed the majority of prisoners deported to the Operation Reinhard killing centers by locking them in stationary gas chambers into which truck engines pumped deadly carbon monoxide gas. A minority of prisoners were killed by shooting.

...In the Operation Reinhard killing centers, the SS and their auxiliaries killed approximately 1,526,500 Jews between March 1942 and November 1943.

Germany began "deporting" its own Jews in 1941, first to ghettos, and then to camps and killing centers.

German authorities began to deport Jews from the Greater German Reich in October 1941, while the construction of the killing centers was still in the planning stage. Between October 15, 1941, and November 4, 1941, German authorities deported 20,000 Jews to the Lodz ghetto. Between November 8, 1941, and October 1942, German authorities deported approximately 49,000 Jews from the Greater German Reich to Riga, Minsk, Kovno, and Raasiku, all in the Reich Commissariat Ostland (German-occupied Belorussia, Lithuania, Latvia, and Estonia). SS and police officials shot the overwhelming majority of the deportees upon arrival in the Reich Commissariat Ostland.

German authorities deported another approximately 63,000 German, Austrian, and Czech Jews to the Warsaw ghetto and to various locations in District Lublin, including the transit camp-ghettos at Krasnystaw and Izbica and the killing center in Sobibor, between March and October 1942. German Jewish residents of the Lodz and Warsaw ghettos were later deported with Polish Jews to Chelmno, Treblinka 2, and, in 1944, to Auschwitz-Birkenau.

Here is a photograph of Zyklon B pellets found at the liberation of the Majdanek camp in Poland, after July 1944.

The Nazis also used mobile gas vans.

Here is an overview of the Nazi's gassing operations.

Two of every three European Jews died in the Holocaust.

German SS and police murdered nearly 2,700,000 Jews in the killing centers either by asphyxiation with poison gas or by shooting. In its entirety, the "Final Solution" called for the murder of all European Jews by gassing, shooting, and other means.

Six million Jewish men, women, and children were killed during the Holocaust — two-thirds of the Jews living in Europe before World War II.

Sean Spicer should be fired for his ignorance of history.