A.T. Robertson Commentary Romans 16:4

"who for my life laid down their own necks; unto whom not only I give thanks, but also all the churches of the Gentiles:" — Romans 16:4 (ASV)

Laid down their own necks (τον εαυτων τραχελον υπεθηκαν). First aorist active of υποτιθημ, old verb to place under (the axe of the executioner), only here in N.T. in this sense, though in 1 Timothy 4:16 to suggest. If literal or figurative, the incident may be connected with the uproar created by Demetrius in Ephesus. Certainly Paul felt deep obligation toward them .

Not only I (ουκ εγω μονος). Rather, "not I alone" (adjective μονος). The Gentile churches also (great mission workers).