By a journey to Gabon on less than 1 month recommend protection against the following infections:

Diphtheria
Diphtheria is a serious throat infection, which infects from person-to-person through the air. The vaccination should be less than 10 years old otherwise a booster is needed.
Read more about diphteria here.

Malaria
There is considerable risk of malaria and prophylaxis is recommended. The choice is between Malarone, Doxycycline or Lariam. Protection against mosquito bites will reduce the risk further. 
The distribution of malaria can be seen at the map below.
Read more about malaria here.

The information on malaria risk in individual countries are based on the sources found here

Yellow fever
Yellow fever is a virus infection, which cause a serious hepatitis. The vaccine provides protection for 10 years after a single injection. Countries where yellow fever is present are entitled to request a valid certificate documenting a vaccination against yellow fever at least 10 days before entry. The certificate is a stamp in the WHO yellow book.
Read more about yellow fever here.

Hepatitis A (infectious hepatitis)
Infectious hepatitis infects through contaminated food and water. Vaccination consists of two injections about 12 months apart, which protects for up to 25 years. The first vaccination protects for 12 months. 
Read more about hepatitis A here.
The hepatitis A vaccine can be combined with the vaccine against hepatitis B

HIV in Africa
109 Danes were infected with HIV in Africa between 1991 and 99. Africa is the continent with most HIV infected people. Overall about 8% of the population is infected with infection rates reaching up to 80% in risk groups like prostitutes.
Read more about HIV here.

Meningitis
Meningitis due to meningococcus bacteria infect from person to person through the air. The vaccine protects against infection for 3 years after 1 vaccination. There are two vaccines: One protects only against type A and C. The other protects against A, C, W135 and Y and is used for travel to Saudi Arabia (especially Hajj) and West Africa.

Read more about meningitis here.

Tetanus
Tetanus is a complication to wounds contaminated by soil. If there has been a vaccination within the past 10 years it is not necessary to give a booster in case of wounds and accidents. Tetanus vaccinations are usually given in combination with diphtheria vaccine. If you are previously vaccinated, the vaccine can be given right up to departure.

Recommended vaccinations Starting Diphtheria 1 day Malaria 1-7 days Yellow fever 10 days Hepatitis A (infectious hepatitis) 1 week HIV in Africa Meningitis 1 week Tetanus 1 day 10 days When to start vaccinations before leaving::