Was ist das “G” das mich in mein Sitz drückt, wenn ich Gas gebe? Gibt es das im Weltraum nicht? Warum?

Yes, there is also in space.

https://de.wikipedia.org/wiki/G-Kraft

g– Forces loads which act on the human body, an object of use or a vehicle due to a severe change in size and/or direction of speed. In the event of loads on technical devices such as aircraft or the indication of load limits, the term
Frequent used.
^[1] The
g– Force for one
Force per
Mass, it has
Dimension a
Acceleration and becomes a multiple of
Case accelerationg specified. High
g– forces occur, for example, during trips with
roller coasterat rocket launches in which
Washing machine when spinning or
Clashes of objects.