Gauff, 18, and Garcia join world number one Iga Swiatek, Ons Jabeur and Jessica Pegula in the singles draw for the finals in Fort Worth, Texas from October 31 to November 7.
Gauff will also play in the doubles with partner Pegula, making them the first Americans to take part in the WTA Finals in both singles and doubles since Serena and Venus Williams in 2009.
🇺🇸 𝗖𝗼𝗰𝗼 𝗶𝘀 𝗶𝗻! 🏁✅ Singles✅ Doubles with @JLPegula ✅ The youngest player to compete at the #WTAFinals since 2005… https://t.co/ZjyESWXVfl
— wta (@WTA) 1666214068000
At 18 years, 239 days old at the end of the event, Gauff will be the youngest to take part since Maria Sharapova reached the semi-finals in 2005, aged 18 and 208 days.
Gauff, who moved into the last 16 of the Guadalajara Open with a straights-set win over Elisabetta Cocciaretto on Wednesday, has had a stellar 2022, winning three titles and reaching the semi-finals of the US Open.
Garcia, 29, returns to the finals for the first time since 2017 when she reached the semi-finals.
The top seed in Guadalajara, Paula Badosa of Spain, retired in the second round on Wednesday having lost the first set to two-time Australian Open winner Victoria Azarenka.