Brady and Gronkowski have yet to take part in OTAs this offseason.
Both Brady and Gronkowski considered retirement this offseason, but they each came forward and said they would play in 2018.
Although Brady and Gronkowski weren't present for the start of Patriots minicamp, Karen Guregian of the Boston Herald reported they both received treatment at the nearby TB12 training center Monday.
TB12 is run by Brady's personal trainer, Alex Guerrero.
Last year, ESPN's Tom Junod and Seth Wickersham reported that Patriots head coach Bill Belichick had an issue with Guerrero, which resulted in him no longer traveling with the team or being permitted to treat players other than Brady at team headquarters.
Brady, who has missed OTAs in the past, is coming off an MVP season at the age of 40.
Gronkowski suffered multiple injuries last season, including a concussion that forced him to miss much of the AFC Championship Game against the Jacksonville Jaguars.
On Tuesday, Belichick refused to discuss Brady and Gronk's absences, saying, "I'm not going to talk about the people who aren't here," per Rachel Bowers of the Boston Herald.
While the current minicamp is voluntary, mandatory minicamp will take place from June 5 to June 7.