The Cambridge Dictionary defines “the gift of the gab” as “the ability to speak easily and confidently in a way that makes people want to listen to you and believe you”. If you feel nervous when speaking and you can’t speak easily then yes, you could say “I don’t have the gift of the gab”.

so you want to use sarcasm to lower the tention, nice! but you can also be straightforward and say: “sorry i get a little nervous around people”. they will probably encourage you and you will feel more relaxed. second, maybe you lack selfconfidence, i had the same problem until i stopped and listened to other people around me talking and, they sucked! but they just spoke anyway, god bless them. they even invented words but being confident makes the difference.

Someone who has 'the gift of the gab' is someone who just likes talking, even if they make mistakes or have difficulty explaining themselves, so it doesn't really apply here. Also, if you say to people 'I don't have the gift of the gab' it might sound like you are not interested in talking to them. I often find that when you're speaking a foreign language to native/proficient speakers, they don't pay attention to the mistakes that you make, they just focus on the conversation. In fact, I find that people generally have a lot of respect for others who are able to speak their language. Maybe instead of telling people in advance that you don't want to talk, you could say 'sorry, I get a bit tongue-tied in English' when you feel like you're not expressing yourself correctly.