not sure if usual gp can provide letter but my gynae was able to provide a letter to help me request for work from home due to pregnancy to reduce chance of covid and due to morning sickness etc! i asked for it around 12 weeks. she also provided another letter stating EDD which will be useful for maternity/paternity leave (or even deferment of reservist if it falls around EDD) my managers actually considered sending me for business trip when i was around 8 weeks, so i just told them about my pregnancy early. They were pretty understanding and didnt ask for letters or anything.
Wonder if boss can waive off MC and allow you to be excused due to your condition? If you're unwell with morning sickness, it's hard for you to enjoy the retreat too
be honest with your direct manager. most will be understanding. you can definitely get a letter from gynae to certify your pregnancy and also your morning sickness.