Early Germanic people were related in the first place by the same mother tongue, in the second place, by being members of the same group of tribes which was expressed by close cultural/lifestyle things.
I have seen the Y DNA from most Germany on the FB group and I think that assuming "West Germanic people carried mostly I1 and R1B-U106" is not entirely correct.
Most areas of Germany do not have significant I1.
All areas of Germany have R1B-U106,raising to even 29% in Lower Saxony.
In Lower Saxony I1 is 28% and I2B is 10%.There is also I2A, at 3%.
East Frisia got 24% I1, and 14% I2B.Other areas of Germany, do not have over 20% I1 .
East Pomerania, where Goths should have been, sometimes, have 21% R1A-Z280 and 7% R1B-U106. More strange results for East Pomerania, R1B-DF27 - 10%, R1B-L21 - 10%.I2a- 6%. R1A makes most paternal lines in East Pomerania, 35%.
In regards to East Germanic people, no one can say for sure what Y DNA the Goths had. In Spain, where more Goths settled, there is a higher concentration of R1A. In Romania, where should be plenty of Y DNA that remained from Goths, no one bothered to do Y DNA testing.
To come back to the first part of the post:
I doubt that the migration of West Germanic people was a fast one.
If for example West Germanic people acquired some land near and assimilated locals, the Y DNA was changing.
The assimilated locals become West Germanic also. Now, the West Germanic tribe which consisted of the original people and the assimilated locals move to take under their rule a new land. And so forth.
Is quite clear from the East Germanic and West Germanic languages that the males of the acquired lands were not replaced.
So this is how the Y DNA of West Germanic and East Germanic people seems to have been quite diverse.
For example, in Prussia there is 24% N1 Y DNA.
Since the Ancient Prussians, which were BaltoSlavic, were assimilated, not exterminated.
Another observation, most Germany has some I2B and most Germany has some I2A, even few.
I2B seems to be a marker of West Germanic people and of Gothic people, but at low percentages.
EDIT:Sorry, East Pomerania belongs to Poland now and East Prussia, to Poland, Russia,Lithuania.
EDIT2: The Y DNA data is from the time when East Pomerania and East Prussia were part of Germany and are obtained from the DNA of the people that had male ancestors Germans from East Pomerania , East Prussia.
This is how it seems.