Can a pharmacy and laboratory do sti screening or i have to visit the gp first?

See your doctor: You should be examined by your doctor to ascertain if you need to be tested and what infections you should be tested for.

Be careful: In some developing countries (much of Africa, Asia, parts of Latin America), pharmacies do a big business selling drugs for STIs based only on symptoms. Sometimes good advice but often wrong, resulting in treatments that don't work best and might even be harmful. It's even more difficult to generalize about quality of lab testing in pharmacies. Far better to visit a doctor or clinic.