Gargnano is a cute lake town and it's in the north lake, which is my favorite part.
You don't say if you have a car, therefore I assume you don't have one and I'll give you the public transport information. With a car it's just easier because you don't have to worry about schedules.
Other towns to visit are (just giving you a few must see)
Limone and Riva (both are on the same shore as Gargnano, therefore faster by bus)
Malcesine (on the opposite shore, therefore maybe faster by boat, although you can go around the lake on the north side (via Riva) by taking two buses (change in Riva).
On the south side the most famous town is Sirmione. It's by far the most visited town on the lake, therefore expect Disneyland type of crowds on weekends, especially in Spring/Summer. However it's a must see, therefore if you can, at least avoid visiting on weekends to avoid the crowds. Sirmione is probably easier by bus, via Desenzano (a larger town on the south side).
The lake is serviced by 3 bus agencies. One for each of the 3 provinces surrounding the lake (Brescia, Verona, Trento). Gargnano is in the Brescia province and is served by the respective agency.
The bus schedules for all 3 agencies are below:
http://www.lagodigarda.it/Linee-autobus/1158-1.html
For boat transportation the ferry schedules are below. However also consider that in addition to these regular boat lines there are also several private boat operators that operate boat excursions for tourists. Prices vary but are not necessarily much higher than the ferry lines below:
http://www.navlaghi.it/eng/g_illago.html