Map from Google - La_Gomera

In La Gomera, a small round shaped island belonging to the Canaries, the climate is subtropical oceanic on the coast, that is, very mild and sunny most of the year, with very mild and spring-like winters, and warm and sunny summers, and also with little rainfall, concentrated in the period from October to March.
In the interior, the climate varies according to altitude and exposure: in fact the slopes exposed to the northeast trade winds receive a fair amount of rain and are green, while in the southern slopes the rainfall is scarce, often below 350 millimetres (13.5 inches) per year, to the point that the island appears to be divided into two, the northern part being verdant and the southern one semi-arid.
The prevailing winds, the trade winds, make the northern slope more humid, while tempering the heat along the coasts during summer; the trade winds blow more intensely in the afternoon, being often moderate or even quite strong. The days of calima, characterized by abnormal heat and light wind, in which warm air masses from Africa prevail, are relatively rare, and are more frequent in spring and summer: in practice these are the only days in which the temperature exceeds 30 °C (86 °F), and in which the heat is unpleasant. The calima is also felt on inland hills, and often even more than in the plains, because warm air is lighter and tends to rise, while the coast can be mitigated by the coolness coming from the sea.

Playa Santiago, La Gomera

Along the coasts, therefore also in the main city, San Sebastián de la Gomera, daytime temperatures are very mild in winter, when highs are around 21 °C (70 °F) in January and February, and warm in summer, when highs are around 26/27 °C (79/81 °F) from July to September. Here are the average temperatures in San Sebastián.
Average temperatures - San Sebastián
San SebastiánJanFebMarAprMayJunJulAugSepOctNovDec
San SebastiánJFMAMJJASOND
Min (°C)161516161719212121201817
Max (°C)212121222324262627262422
Min (°F)615961616366707070686463
Max (°F)707070727375797981797572

The rain pattern is Mediterranean, which means that the bulk of the rain falls between October and March, because it is the season when the Canary Islands can be reached by some Atlantic depressions; in summer, on the contrary, in practice it never rains.
Here is the average precipitation in San Sebastián.
Average precipitation - San Sebastián
San SebastiánJanFebMarAprMayJunJulAugSepOctNovDecYear
San SebastiánJFMAMJJASONDY
Prec. (mm)60504025841211406570376
Prec. (in)2.421.610.30.200.10.41.62.62.814.8
Days66653211356751

The interior of the island is entirely occupied by hills and mountains: here the weather gets cooler as you go up in altitude, and as mentioned the north side has a cool and humid microclimate, with rains of some importance in the cold half of the year, and also more frequent mists and clouds for much of the year, due to the trade winds, so that there are several towns that are situated in a green environment. At 600/700 metres (2,000/2,300 feet), the daytime temperatures are about 16 °C (61 °F) in winter and 25 °C (77 °F) in summer.
The highest peak is Garajonay, 1,487 metres (4,879 ft) high. The area to the north of the mountain is very green, being covered by a thick forest of laurel trees, besides it's home to several endemic species, it is a national park (called Garajonay National Park), and also a World Heritage Site by UNESCO since 1986.

View from the top of Garajonay, Mount Teide in the background

The amount of sunshine on the coasts of La Gomera is good even in winter, and is excellent in summer, when clear skies are the norm. In inland areas instead, especially on the slopes facing north and east, mists and clouds may form quite frequently.

The sea in La Gomera is quite cold in winter, though not prohibitively, and becomes warm enough for swimming from August to October. The island is located at a low latitude, near the Tropic of Cancer, but in this part of the Atlantic a cold current flows. The sea temperature goes from 19 °C (66 °F) in the period from January to April, to 23 °C (73 °F) in August, September and October.
Here are the average sea temperatures.
Sea temperature - La Gomera
La GomeraJanFebMarAprMayJunJulAugSepOctNovDec
La GomeraJFMAMJJASOND
Sea (°C)191919192021222323232220
Sea (°F)666666666870727373737268

When to go

For swimming and sun bathing, the best time to visit La Gomera runs from May to mid-October. In the first period (May-June), the air temperature is a bit lower and the sea is a bit colder, but the days are slightly longer. In the Canaries, in summer the days are shorter than in Europe (while in winter they are longer), but due to the time zone adopted, the sun sets late enough anyway.
To visit cities and for excursions to the interior, you can choose April, May and the first half of October.
In winter you will appreciate the mild temperatures and you can often sunbathe, although we can not exclude some days of bad weather.

In suitcase - what to bring

In winter: light clothes for the day, sweater and jacket for the evening, possibly a raincoat or umbrella. To climb Garajonay, down jacket, hiking shoes.
In summer: summer clothes, but also a scarf for the wind, a light sweatshirt, a light jacket for the evening and for windy afternoons; comfortable shoes for hiking. To climb Garajonay, hiking shoes, sweathirt and light jacket.