Map from Google - Gran_Canaria

In Gran Canaria, 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 mild, spring-like winters, and warm and sunny summers, and also with little rainfall, concentrated in the period from October to March. In inland areas, however, 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 on the southern slopes rainfall is scarce, often below 150 millimetres (6 inches) per year, to the point that the island appears to be divided in two, the northern part being verdant and the southern one almost desert.

Valleseco, Gran Canaria

The prevailing winds, the trade winds, make the northern slopes more humid, and also temper the heat along the coasts during summer; these winds blow more intensely in the afternoon, when they are often moderate or even quite strong. The days of calima, characterized by abnormal heat and light wind, due to warm air masses from Africa, are relatively rare, and are more frequent in spring and summer: in practice these are the only days when the temperature exceeds 30 °C (86 °F), and when the heat is unpleasant. The calima is felt also on the highlands, and often even more than in the plains, because hot air is lighter and tends to rise, while the coast can be mitigated by the sea.

Along the coasts, and therefore also in the major cities, such as Las Palmas de Gran Canaria, Ingenio, Telde, the daytime temperatures are very mild in winter, with highs around 21 °C (70 °F) in January and February, and warm in summer, with highs around 27 °C (81 °F) from July to September. Here are the average temperatures of the airport of Gran Canaria, located in the eastern part of the island, near Ingenio.
Average temperatures - Gran Canaria
Gran CanariaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)151515161719202121201816
Max (°C)212122222325272727262422
Min (°F)595959616366687070686461
Max (°F)707072727377818181797572

The rain pattern is Mediterranean, which means that the bulk of the rain falls between October and March, because it is the time when the Canary Islands can be reached by some Atlantic fronts; in summer, on the contrary, in practice it never rains. The total amount of rain that falls in a year is still very low, below 200 millimetres (8 inches), therefore at desert levels. Even in autumn and winter, weeks can pass without a drop of water falling, although occasionally there may be more intense rains, as happened in the past from October to February.
Here is the average precipitation.
Average precipitation - Gran Canaria
Gran CanariaJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)1824147200010131827133
Prec. (in)

In inland areas 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 a certain amount from March to October, and even mists and cloudiness are more frequent, 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 Pico de las Nieves, located in the centre of the island, and 1,949 metres (6,394 feet) high. At this altitude in winter it's cold, so much so that highs hover around 10 °C (50 °F) and lows are just above freezing (0 °C or 32 °F); sometimes the temperature can drop below freezing and even snow can fall; in summer the temperatures are pleasantly warm during the day (but the sun is very strong, so it requires adequate protection like sunglasses and hat), and cool at night. From the top of the mountain you can see Mount Teide, the highest peak of Tenerife and all the Canary Islands.

Landscape of Pico de las Nieves, Teide in the background

The amount of sunshine on the coast of Gran Canaria is good even in the winter months, and is great in summer, when clear skies are the norm. In inland areas instead, on north-facing slopes, mists and cloud banks can form with some frequency.
The sea in Gran Canaria is quite cold in winter, though not prohibitively, and stays cool in summer, as the island is located at a low latitude, almost at the Tropic, but in this part of the Atlantic Ocean a cold current flows. Those who live in the island usually do not stop swimming in the sea even in winter, being accustomed, and also Nordic tourists or those who do not suffer from the cold. Anyway, the sea temperature goes from 19 °C (66 °F) in the period from January to April, to 23 °C (73 °F) in September and October.
Here are the average sea temperatures.
Sea temperature - Gran Canaria
Gran CanariaJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)191919192021222223232220
Sea (°F)666666666870727273737268

The capital, Las Palmas, is located in the north-east of the island, where also the beach called Playa de Las Canteras is located; other famous beaches are located in the east and south of the island, where there are also the Maspalomas Dunes, a small sand desert along the coast.

Gran Canaria, beach

When to go

For the sun and the sea, the best time to visit Gran Canaria goes 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.
To visit cities and make excursions to the interior of the island, 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. If you want you can combine a trip to Pico de las Nieves, in search of the snow, with an afternoon on the beach.
On the other hand, weather disturbances can bring high waves generated by winds different than the typical trade winds, and that can be appreciated by surfers.

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 Pico de las Nieves, down jacket, hat, gloves, hiking boots.
In summer: summer clothes, but also a scarf for the wind, a light sweatshirt, light jacket for the evening and windy afternoons; comfortable shoes for hiking. To climb Pico de las Nieves, hiking shoes, sweatshirt and jacket for the evening.