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.

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 single-argument OptionValue expression must appear literally on the right-hand-side of a rule or definition with OptionsPattern, which your use of Evaluate does. 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.

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. Nevertheless you can, for example, 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.

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.