The deciding clue is the time expression. Last summer is a definite, finished past time, and English requires the simple past whenever the time of an action is both stated and over.

The simple past of go is went, so ‘I went to Germany last summer.’ is correct.

Why each of the others fails:

‘I go…’ — simple present, which describes habits or general truths and clashes with a finished past time.

‘I am going…’ — present continuous, which describes an action happening now or a planned future arrangement. Neither can sit with last summer.

‘I have gone…’ — present perfect. This is the important one: the present perfect is never used with a definite past time expression, because it links past to present rather than fixing the action at a stated moment. ‘I have gone to Germany’ alone is fine; adding last summer makes it wrong.

Compare: ‘I have seen that film’ (no time stated — correct) against ‘I saw that film yesterday’ (time stated — simple past required).