It would help if you outlined you intended use of this behavior, as without that it is not clear what is and is not helpful.
Single function case
You can use the two-argument form of OptionValue:
rule = {"t" :> OptionValue[gg, "t1"]}; (* note RuleDelayed *)
Options[gg] = {"t1" -> "T1", "t2" -> "1"};
gg[___, OptionsPattern[]] := StringReplace["content", rule]
gg[1]
"conT1enT1"
This works just fine with a single function (gg), but it is not directly applicable if you intend to use this rule in multiple functions.
Arbitrary function case
As you apparently understand based on your workaround, the OptionValue expression must appear literally on the right-hand-side of a rule or definition with OptionsPattern. Your use of Evaluate causes the OptionValue expression to appear explicitly for SetDelayed. Any other method that does the same can be used, e.g.:
With[{rule = rule},
gg[___, OptionsPattern[]] := StringReplace["content", rule]
]
Or:
(gg[___, OptionsPattern[]] := StringReplace["content", #]) & @ rule
For the arbitrary-function case I see no simpler method than these.