The Garmins (and I suspect other manufacturers) are supplied with a Jeppeson database of aeronautical information, which is more or less current at the time of production (with 1 free update on purchase). This provides airports, runways, frequencies, navigation aids, airspace volumes etc. for a given area (e.g. Atlantic area = Europe and Africa).
However, the aeronautical data is subject to change every month according to the AIRAC cycle, so after one month some of the aeonautical data in your GPS is out of date, and you are probably missing some new data.
You can buy monthly updates from Jeppeson, through the Garmin site for around $40 a month, or a 13 month susbscription for +/- $300.
Whether you need to do this or not is a matter of choice - how much money do you have, what do you want the GPS aeronautical data for, and what has changed in your most-likely flying areas that you need to be updated in your GPS? Getting updates because of changes to ILS frequencies in Angola is maybe not the best of reasons. However, if the Birmingham TMA and CTRs have changed significiantly that might justify an update.
The "VFR GPS" aeronautical data, especially if not updated, should not be your first port of call for aeronautical information such as nav. aids, frequencies and controlled airspace. You can cross check your PLOG and the GPS to see if the aeronautical data corresponds.
If your current database properly reflects the area you are planning to fly in, then there should be no problem.
I took out a 13 month subscription but then decided it was too expensive. Now I update about 2 or 3 times a year (= 1 hour of flying).
(edited to add that nav. aid locations don't usually change much from month to month
- however, airport frequencies can be more variable).
GB