I know that
geradeaus
means straight. But I also heard its variation:
gradeaus
Is it appropriate to say or write this word like this?
Its meaning is the same.
Gradeaus
is a kind of idiom. The written word is always
Geradeaus
both words are translated to straight.
"Gradeaus" is a contraction for "geradeaus."
Think of geradeaus spelled with an apostrophe instead of the first e: g'radeaus.
In spoken language unstressed e often is omitted. You can hear gerade, grade and grad. It's all the same. But you write gerade.
It also depends on the location you are in. In South Germany, they say gradeaus, but in the north, they say greadeaus.