Bubblegum pop is usually characterised by (often quasi-sexual) references to food or candy, notable examples being The Ohio Express’s “Yummy Yummy Yummy (I Got Love in My Tummy)” and “Chewy, Chewy,” and The Archies’ “Sugar Sugar.” Other childhood themes are common, such as children’s games (e.g. “Simon Says” and “1, 2, 3, Red Light” by The 1910 Fruitgum Company).

Bubblegum is often not really the shallow, synthetic pop most people think of when they hear the name. Many of the groups in the genre would perhaps be better described as “garage rock” or even “punk.” All of these descriptors are mainly ways of differentiating these groups from the progressive direction of most rock music of the time. Both bubblegum and garage rock were musical forms that maintained their adolescence/childishness, frequently to the point of self-parody. The first Kasanetz/Katz-produced group The Music Explosion is a great example of this, with powerful garage tunes such as “Little Black Egg” (with a brilliant punk cover ten years later by fellow Ohioans, The Pagans).

Other examples of more garage-y sounding bands are Captain Groovy and His Bubblegum Army with “Bubblegum March,” their pop take on an acid rock instrumental, and The Shadows of Knight. Some bubblegum ventured into goofy psychedelia, such as The Lemon PipersGreen Tambourine.”

The 1980s saw a resurgence in bubblegum-esque pop from such new wave groups as The Buggles, Blondie, Bow Wow Wow, The Bangles, Cyndi Lauper, and The Go-Go’s.

The main bubblegum label was Buddah Records, which put out albums by The Ohio Express, The 1910 Fruitgum Company, The Lemon Pipers and others.

Performers/Groups often classed as “bubblegum”:

The Boyce and Hart Group
(Tommy Boyce and Bobby Hart being the Monkees’ primary songwriters)

The Candy Store Prophets
(Boyce and Hart's backup group, they sometimes recorded as The Monkees, e.g. "Last Train to Clarkesville.")

Kasenetz-Katz Singing Orchestral Circus
(AKA Kasenetz-Katz Flying Super Cirkus)

The Rare Breed

The Music Explosion

Tommy James and The Shondells

The Monkees

The Archies

The Shadows of Knight

Captain Groovy and His Bubblegum Army

The Ohio Express

The 1910 Fruitgum Company

The Lemon Pipers

The Osmonds

Tommy Roe

Bobby Sherman

Tony Defranco and the DeFranco Family

The Cowsills
(a family act that nearly had a television series but were instead replaced by The Partridge Family.)

The Partridge Family

The Royal Guardsmen
(famous for their hit “Snoopy vs. the Red Baron”)

Let me chime in with General Wesc's sentiments: Though bubblegum can certainly, in many cases, be criticised for being shallow, it is silly to differentiate it from other pop music for being too commercial, or indeed for being synthesised. Many more reputable popular performers haven't written their own music, and nearly everyone's sound is more or less influenced by their producers.
Please message me to suggest any additions/corrections to this writeup.