Seat 1 = William
Seat 2 = Nick
Seat 3 = Eric
Seat 4 = Sydney
Seat 5 = Gabbie
Seat 6 = Amanda
Seat 7 = Sarah
Seat 8 = Alisa
Sydney is in Seat 4 and is next to Gabbie (Clue 6), so Gabbie is in either Seat 3 or Seat 5. But Gabbie has a higher seat number than Sydney (Clue 7). Therefore Gabbie is in Seat 5.
William is across from Gabbie (Clue 2) and therefore is in Seat 1.
William is next to Alisa and Nick (Clue 1), so Alisa and Nick are in Seat 2 and Seat 8 (either way around). But Alisa is not in Seat 2 (Clue 8). Therefore Alisa is in Seat 8 and Nick is in Seat 2.
Nick is next to Eric (Clue 5) so can only be in Seat 3.
Sarah is across from Eric (Clue 4), and therefore is in Seat 7.
Amanda is next to Sarah (Clue 3) and can only be in Seat 6.